I don't know, because I keep trying to marry my fiance but he keeps shooting down all of my inspired location ideas.
That is a terrible inside joke that you're not privy to, I apologize. But to give an actual answer that you've probably already figured out from what others have said, it's really varying. There's a lot of reasons like love and devotion, but back home we also had noble families that married to make those positions of power stronger or other sorts of marriages of convenience, I suppose. It's a legal contract, end of the day, and it's up to the people who sign it if it's anything more or less than that.