A couple of posts I've seen on here today brought this issue to mind.
Those are 5 very reasonable signs of an abusive relationship, and something I have seen many times among friends and family. When I see these signs I will tell the person(mostly the woman) that I see them, today I can't say that even one of them listened and many of these type of relationships end only to have the person enter into the same type of relationship the second time.
This does baffle me seems that they keep seeking what they know.
It's a learned behavior. You don't think you deserve better/aren't aware that other forms of relating exist. I've seen men and women end up in those situations and it only ends when they start treating themselves differently. It's hard to learn to value yourself more than a relationship or SO when you've been convinced that's the only way someone is ever going to "love"you.