Because somtimes women are masochists (I hope that's the right word) and like drama and turmoil in their lives. Somtimes they are used to pain and drama so much that's all they know. My ex used to treat me very bad but sometimes I wonder if I would go back with him if I had the chance, and sometimes my response scares me. I think I'm used to drama and being emotional and arguing all the time that in some sick twisted way I thrive off of it. I need help
Sometimes that's all a person knows so they kind of expect to be treated badly in a relationship. Personally I think it comes from self-esteem, self-worth issues.