Why?

One of my friends called me the other night. She'd just found out that her eighteen year old pregnant niece is being abused by her boyfriend, the father of her child. Evidently, he had been hitting her in the stomach among other places. Luckily the neighbors called the police, and they arrested him and took her to the hospital despite the fact that she didn't want to go and tried to tell them that she had fallen. It's apparently not the first time he's hit her. This was all news to my friend and she was furious (of course), and extremely suprised. She was finding it hard to believe that her niece would have put up with this type of treatment, gone back for more and then tried to protect her abuser. This is all typical behavior for a battered woman and she knows that, but when it's someone you know it's very difficult to comprehend the "why" of it.
I have a cousin, who was in the same type of abusive relationship for years with her husband (now her ex.--thank God!) and I just couldn't fathom why she stayed. I'm sure low self-esteem, and that belief that "he really does love me", have a lot to do with it. Fortunately, I don't have any first hand experience in that arena and perhaps that's why I don't totally get it. Frankly though I think if someone tried that with me-let's just say they better never sleep again!