I do this too sometimes, stick around people who make me feel crappy. I think it's because I don't have a ton of people in my life and the people who do that are sometimes friendly with me, you know? I don't have the social intelligence to understand when they are being jerks to me until way after I've already been used or hurt or something. I don't really fall into this trap so much anymore, but I used to in my twenties all the time. I think it kind of had to do with a mutual co-dependence of sorts. Maybe.