When you're single, the thing you want most in the world is a great relationship. When you're in a great relationship, the thing you want most in the world is to mess it up and be off the hook.
OK so that's a generalization, but how else can you explain why people go to such crazy lengths to create drama in their relationships when they could otherwise be great? Or, and this is a lot more common: why is it that people rush to get into relationships that they knew from the beginning would be a nightmare to deal with?
I don't know, but if it's any consolation, humans have been dealing with relationship issues since the dawn of time. I mean, why else would these captions on classical paintings fit so well, then?