Why do couples do stupid things to each others?

When one cheat on their partner, it angers me. I barely get boyfriends and couples who do things like that, it's barmy. Some people don't know how lucky they are to be in a good relationship. I understand why one partner cheat on their partners but it's bad, it's cowardice you know? It leaves me a big mess behind that sometimes is hard to clean up after oneself.