It's the same reason gambling is so powerful as an addiction. It's called loss aversion, and the sunk cost fallacy. Let's say you've been in a relationship for about a year and things have been great. You love her and she loves you, and then the honey moon phase sort of ends and the real demons start coming out one by one. The bad habits, the insecurities, and the political ideologies come out of the wood work and you don't like a damn thing, hell she seems like a completely different person. But you had that one good year, you put a lot of effort into this relationship and your not about to let a couple of little things like that end a long and steady relationship. Here's the catch though the longer you go the harder it becomes to end it because of the time and effort already put in. Meanwhile your relationship starts turning sour because she's been having the same thoughts about you but neither of you can honestly imagine what life would be like without the other. At this point the only way it can end is horribly
Because if they weren't, a commited relationship would be effectively meaningless and no secure bond could be made. Humans are social creatures and pair bonding is part of our evolutionary history, gained as a result of having parents bonded together in a relationship being better at raising children successfully. If breaking up wasn't rough, it wouldn't have resulted in successful babies, so that trait was naturally discouraged since the people who could easily break up didn't have as successful offspring.