Caleb*, a photographer, and Mike*, a musician, both 33, met at an arts festival and became fast friends, eventually laying down roots just blocks from each other in New York City. When Caleb slipped into a scary bout of depression, he leaned on Mike. "We saw each other almost every day," recalls Caleb. "I was having a hard time being alone, so I followed him around. He tried to entertain me, and we discussed my fears about what was happening. I felt like he compromised his own life because he was spending so much time on me."

After Caleb came out of his depression, the two drifted apart. Caleb thought Mike would appreciate the break; he also started dating a woman Mike disapproved of. Caleb's efforts to keep the two apart meant that the once-inseparable friends spent even less time together. "Mike read that as me ditching him when he was no longer needed."

The unspoken rift kept them out of touch for almost two years. Finally they patched things up, but only after Caleb received "a pretty big lecture about what I'd done by 'disappearing.'"

Solid friendships make us happier and healthier, and in our late-marrying, highly mobile society they're more important than ever as pillars of support. But like any human entanglement, they can cause pain and confusion. For every Sam-and-Frodo tale of loyalty and sacrifice, there's a Brutus or Judas who breaks a heart. And there are myriad situations in between.

1: Growing Apart

"I feel awkward and self-conscious sharing my life with her, and I'm not that interested in hers. When we were teenagers, we'd stay up all night talking."

Longtime friends can be invaluable, as they often understand your mosaic of experiences and emotional makeup. But how do you sustain a relationship if your primary-school bosom buddy is off reforesting the developing world while you are prancing around parties in stiletto heels?

We tend to choose friends based on who's close by and similar to us. The question, according to Judith Sills, a Philadelphia-based clinical psychologist, is whether you can accept the trade-offs that occur when friends individuate. "The more heterogeneous our circle ," says Sills, "the more rewards, the more access, and the more opportunities we reap. We also reap more discomfort, though. So expect to feel annoyed or threatened on occasion when you have a diverse group of friends."

Friendship is accordion-like: Sometimes you'll be close, sometimes you'll be distant, and sometimes you'll revert back to being close. Or not. It can be heartrending to realize that a friendship has petered out. That's why Terri Apter, a psychologist at the University of Cambridge, recommends confronting small conflicts head-on, to avoid a schism. If you're hurt that your friend didn't invite you to her barbecue or tell you about her promotion, say so (in a non-defensive tone) before built-up resentments taint all of your interactions.

And examine your own changed feelings. Let's say a single woman's best friend gets married and has a son. The new mom blows off her friend's e-mails, cuts short her phone calls, and shows up late to coffee dates before holding forth for 30 minutes on sleep training methods. The single woman gets critical: "She's so boring now." But really, she's feeling neglected. It would be better, Sills says , if she were to think, "My friend is not as available to me as she used to be, but life is long. Someday I may have kids, and I'll want her understanding then." When you do reconnect, you'll both bring new experiences to the interaction.

The Takeaway: Accept that friendships erode but then sometimes rebuild. Communicate hurt feelings. Appreciate chums who aren't like you. Wouldn't a life filled with clones bore you?

2: Lending a Hand to A Friend in Need

"I'm sorry to show up at this hour, but I don't know what I'd do without you. I'm so upset."

It feels good to comfort a friend who is stressed out, lovesick, or embroiled in family conflict. We like solving other people's problems—it's certainly easier than facing our own. But if a pal's bad day stretches into weeks or months, how can you help? "Clearly communicate that you are there for her," says Beverley Fehr, author of Friendship Processes. Listening is usually more appreciated than giving advice, and while men tend to appreciate practical support in hard times (rake his lawn, pick up some groceries), women value emotional support (give a hug and point out how well she's taking it all).

Help can easily shade into unwelcome interference, another reason you're better off just listening. When it comes to your friend's love life, for example, she's usually not going to listen to you skewer a new relationship: "Romance is based on a mild positive delusion," points out Nando Pelusi, a New York City-based clinical psychologist. "Your friend will just marshal evidence against your argument."

Friends often fall into respective roles of "saint" and "sinner," Sills observes. The sinner reels from crisis to crisis, reporting all the ensuing drama to her friend, while the saint dependably, patiently, takes it all. If the saint enjoys her

Mother Theresa complex, there's no problem, but if her frustration outweighs the gratification she gets from being a confidante extraordinaire, she needs to set limits. If the sinner is constantly whining about her job but refuses to look for another one, the saint could say, "You know I'm happy to hear from you, but it's clear that you're not ready to leave this job. I keep getting caught lecturing you and I don't like that, so let's not talk about that topic anymore."

If your pal is not merely down about a specific event, but clinically depressed, helping her will be a tough endeavor, because the disease itself makes her resistant to taking action or believing that life can improve. "You'll feel like you are pouring help into a black hole," says Apter.

Your friend will likely need professional guidance to feel better, but just being there is important and soothing—even if it doesn't cause a discernible positive change in your friend, says Martha Manning, a clinical psychologist and author of Undercurrents: A Life Beneath the Surface, a memoir of her own severe depression.

"Friends think: 'I know this person really well. If I try hard and I try a lot of things, I can bring them back,'" says Manning. But people set on "fixing" a depressed friend risk making her feel even more helpless. "Don't presume that what you do will be a cure. You wouldn't presume that if it were pneumonia."

Be pragmatic: Take your friend's kids to school, bring over casseroles, or clean her house. Close friends, says Manning, can gently suggest beneficial activities, such as exercise. But don't push it. "You need to show humility in the face of this disease."

The Takeaway: Think before pulling an intervention. Listening is usually the friendliest thing you can do. But don't let yourself become a limitless dumping ground for kvetchers, either. If a friend is depressed, show up and pitch in, but don't expect to make it all better.

3: Blending Your Pals with Your Partner's

"I can't stand his college friends."

They're a bunch of snobs. I can tell they're judging me." people come in packages, friends and family included. You'll instantly connect with one or two of them, you'll gamely tolerate some, and you might hate a few. Your partner shouldn't automatically drop people you don't happen to like, says Sills, nor should you stop seeing pals that he doesn't care for. Instead, "everyone has to move over a little bit."

Meeting admirable friends of your partner burnishes his reputation. But disliking most of his pals is not automatic grounds for ending the relationship. "They may be people he just happened to room with or work with but who are not similar to him," points out William J. Doherty, a professor of family social science at the University of Minnesota. "If they're a bunch of slackers but your boyfriend is in medical school, you don't have any reason to worry. But if your concerns about his friends mirrors your concerns about him and his values, then you should take that as a warning."

Don't be surprised if the two of you start ditching both of your clans for evenings in. When people get into a relationship, friendship circles shrink, especially for men. It's worth making the effort to maintain ties to a few single friends, since double dates often leave one or two people less satisfied emotionally; it's hard to find a couple you'll both click with perfectly. Friends who have known both you and your partner for a while and who see you frequently are natural divorce-busters, Doherty adds. "Ideally, you'll want a cadre of people who are invested in your relationship. They'll sway you toward resolving your marital problems, for the sake of their own social lives and because they'll be able to see both sides of your conflicts."

The Takeaway: Let bonds form naturally between all parties. Don't complain about your partner's friends you don't like. Instead, nurture friends who genuinely like both of you.

4: Getting Chummy With The Opposite Sex

"Well, yeah, I mean, of course I think she's pretty. And we always laugh so much. But wouldn't getting together ruin our friendship?"

All friendships start with a spark of mutual attraction. "Attraction has a libidinal element," says Sills. "But does that mean that opposite-sex friends have to be lusting after each other? Not necessarily."

And if there is some overt tension, it won't automatically threaten the friends' romantic partnerships. With opposite-sex friends, says Apter, "There are often flare-ups of sexual appreciation, but also the safety of knowing that that expression is going to be contained. We get the reassurance that we can still be attractive without the extreme complication of having an affair."

Things get sticky, of course, when your friend wants to turn the fellowship into a romance, and you absolutely do not.

Or when your partner suspects your opposite-sex pal's intentions are less than honorable. Or when the world at large refuses to accept that you are "just friends." These challenges are pesky, but they are not huge obstacles that routinely undermine such friendships.

When you're facing a buddy who wants more, respond to early signs of sexual interest, and ward them off without humiliating the person. Apter suggests bringing up a hypothetical coupling with a clear rebuff, e.g., "You're watching another game? Good thing we're not a couple—I wouldn't be able to stand your sports obsession."

When one friend is waiting for another to suddenly fall in love with him, a nasty power imbalance develops that can threaten the long-term viability of the relationship, Pelusi warns. If you're the piner, Pelusi suggests working through your own sensitivity to rejection so you can deal with the imbalance in an honest and non-dogmatic way. Instead of trying to "fix" the situation, Pelusi says, both people should "shift their expectations of what a friend should be. That way, they'll open up possibilities of deepening the friendship."

Opposite-sex friends can benefit from the inherently different relationship styles that men and women tend to display. "Women's friendships evolve around self-disclosure and personal feelings and emotions," says Fehr. "When men are friends with women, they tend to be more open and self-disclosing than they would be with a man. A lot of research shows that regardless of gender, the more we pursue closeness, the happier we are in that friendship." For their part, women can learn how to relax and be more spontaneous from their male pals.

The Takeaway: Don't shy away from opposite-sex friendships. A tame flirtation is a psychic boost; you can handle its challenges, and you'll each learn from keeping mixed-sex company.

5: Mixing Business with Buddies

"I should invite Alan to the fairway so I can find out more about the Peterson account. And he'd make a great wingman. Is he just going to think I'm a brownnoser?"

Your boss just "friended" you on Facebook, your top customer's wife invited you to her baby shower (looks like you'll be splurging on a gift for a near-stranger), and your nephew's soccer coach just thrust his resume in your face at afternoon practice: The line between work and leisure is rapidly blurring. In his book, Elsewhere, USA, sociologist Dalton Conley calls this new time weisure. And it's not necessarily a bad thing. Friends and acquaintances of your own friends are your best source for new jobs and clients.

Marketing guru Keith Ferrazzi, author (with Tahl Raz) of Never Eat Alone and Other Secrets to Success, One Relationship at a Time, lays out a central principle for leveraging relationships into career triumphs: Being a sincere, proactive pal is a much better "networking" strategy than handing your business card to everyone you meet. He found that enabling others to achieve their goals quickly improved his own standing in the world.

Here's how it works: Last year Gary helped Jessica get into graduate school; hosted a birthday blowout for Rick at his house; sent out warm, funny e-mails to all of his contacts to promote his brother's new startup; and found the perfect roommate for Erika. Guess who came to the charity event he held on behalf of his organization? Everyone. Not only did they owe it to Gary, they were inspired by his generosity and really wanted him to succeed. The event raised so much money that Gary got a promotion, granting him even more power to help his loved ones.

Don't wait until you're out of a job to call on your contacts, Ferrazzi warns. Keep up with everyone consistently, and connect people you know to others who could benefit from meeting them.

Once you're safely employed, it's a good idea to get close to a few like-minded coworkers. "Friendship is important in every arena, including work," says Sills. "It's emotionally sustaining, and in order to advance, you need allies and support." Survey research shows that 30 percent of employees report having a best friend at work. Those people are seven times more likely than those without a best work friend to feel engaged and happy on the job.

Still, hierarchies and professional codes of behavior complicate office friendships. "We like to think a coworker is a friend—and they could become one,"Pelusi points out. "But even the best intentions can get gravitationally altered by opportunities and competition."

Men are better than women at managing friendship and rivalry, says Sills. "A man can say to his friend, 'I aced you out today!' and they can laugh. Women are more likely to feel uncomfortable after outperforming a friend. If you do feel envious of a pal in your same field, let it motivate you. After all, if you've been managing your weisure time well, your friend's success will ultimately reflect well on you!

The Takeaway: Forget about building an impressive Rolodex. Live to help others, and they'll do the same for you. Form friendships in the office, but brace yourself for the inherent limits and competitive undertones of cubicle alliances.

*Names have been changed.