And don't forget the fear of the other person.
Sometimes the fear is of the reprisals, the repercussions for even thinking that they're better off without the bad half.
I have experience with friends and family who have stayed in bad relationships using the excuse that "he loves me really", despite the philandering and the abuse both physical and mental.
Sometimes I think it's a case of better the devil you know, but more often, I wonder if it's a case of not being able to see the wood for the trees, of being too close to the situation to see what those with an objective viewpoint see...
...or is it just a case of not being able to admit it?