Fear, low self-esteem and confidence, co-dependency, or brainwashed.

Sometimes people don't have the strength to do what's best for themselves. They give up and think it's better than nothing. Maybe the guy is very manipulative and tricks her into it. She thinks he will change. Every situation is different

They feel a similar feel to those people who feel a relatability to their kidnappers, a kind of a Stockholm Syndrome.
They make excuses for the person hurting them.

Low self-esteem, they don't think they are worthy of anything better. Some like the excitement maybe

Some of them feel dependent on them. Like they can't survive without them, in a way. Or they feel like they deserve this sort of relationship, like they can't do better. Or that it's their fault that the relationship didn't work and that they will never find anyone else who will love them.


Abusive relationships can wreak havoc on your mental health and how you see yourself/your sense of self worth.

Sad

It's because they are lonely and haven't found a new boyfriend! Or maybe she got drunk?

good sex ?

Because they are weak and sometimes can't even see it for what it is! If they were strong, they would not put up with that crap. I don't care what they say otherwise. :)