Obsession is nature's way of making cooperation more resistant to random disruptive events.

I think an important skill in relationships is to be able to see a long-term perspective even when you are in a bad moment. Don't ruin a mostly great relationship, just because today is the exceptional day that sucks. It can be easy to start a downward spiral. Forgiving is a way to play "tit for tat" in a noisy environment. The simplest hack to make people forgive is to make them blind towards the mistakes. (Which again comes with its own problems, because evolution is so short-sighted. Some people are too blind; some people forgive too much.)

Costly signalling of cooperation is important in situations where there is so much at stake, such as raising children. I am rather conservative about relationships because... well, if I simplify it a lot, conservatism at its core is all about costly signalling.

Sorry, I'm rambling, because of lack of sleep. So I'll stop now.