what would you do?

Hi, I am new here and wish I can type Chinese on this computer. But anyway... There is something that sometimes bothers me in the office and would like to know your opinion.

I work in a small office, so there are not too many coworkers there. There are two coworkers (a man and a woman) who are very close to each other. But both are married. They go to lunch almost every day. We've been working together for over a year now. In the beginning I did not notice, but gradually I noticed some pattern. Almost every time when the guy talked to me in a nice way, the lady would show an unhappy face and would not talk nicely to me.

After realizing this, I tried to avoid this kind of situation as much as possible. But the thing is we switch teams always, so it is unavoidable that me and the guy work in a same team. What I am trying to do is being as professional as possible. I only talk to him when necessary. It is not such an issue itself. However, with the time I feel this becomes some obstacle for me to develop a healthy working relationship in certain ways. I don't know if you would understand this. Or am I just being too picky and sensitive?

Hope I explained it in a clear way. Has anybody got into this kind of situation?