 Well, I don't really want to spend next Saturday with your parents, but you really want me to be there. So I'll agree to be unhappy for your sake in this case, and that earns me the right to expect that you'll agree to be unhappy for my sake at some future date, when I want you to do something that you don't. And in this way, the couple moves forward into the future, and both participants increasingly spend their time doing things that half the couple doesn't want to do.