Couple lives together because they need each other's help

They take turns as babysitters and share finances. Whether they agree on everything isn't important.

And it's impossible to agree on everything.  It's a social contract so it could be maintained or cancelled freely. So why do  two people choose to stay together after all? One reason: the costs of living apart exceeds the payoffs of staying together. 

For example:  we have three young children, and we're not wealthy, sharing a median income and three children are hard. So none of us can afford taking care of these children alone. It requires two persons' coordination. And we have uneven incomes so one person has to shoulder most of the bills. 

It's basically an ecosystem that exists in nature. It's not love, nor fondness, but only needs that hold us together. One goes down, so does the other. Debt and credit are another factors. It's like two prisoners in a concentration camp that have linked their fates together. Bad credit would ruin them so they got to save each other's ass.

There are no good husband or wife, only two rational adults share a burden of life.