No single reason, more a collective result of a very fucked up relationship. She's a very screwed up person, though the full-extent of that didn't become apparent until we moved in together. She was unhappy with me because I wouldn't conform to her ideals of what I should be as a boyfriend and a person. Instead of compromising, she fucked a few people and we broke up once but we were still living together so it didn't really take because she kept coming back for sex and I kept giving it to her. Then she broke up with me to see some other guy and it seemed to be for good, but then one night she tried to crawl into bed with me again and I kicked her out. Said I woulud never fuck her again.
Since then, she's been doing everything she can to make my life hell.