Fear and lack of self assurance
The feeling that they diserve it. Or they feel they cannot expect to get something better.
Love pulling a person in, the abuse pulling them out. It's difficult to make sense of those two opposite forces.
Low self esteem. They think they deserve it.
The idea that abuse is a form of love that is deserved.
Stupidity ...or mentally challenged
It is not just one thing it is a whole slew of things: doubt, fear, lack of self confidence, love, "I don't want to upset ....".
Fear as well as it being told to them over and over and over.....that they are worthless..that they are the faulty one...that "I really do love you baby, and I'll never do it again"...well till next week when you don't have supper on the table...don't fix what I want...didn't get laundry done...forgot to pick something up at the store......................
Many many reasons....
Security and dependant.
I can think of several. Children first of all.
lack of self-esteem combined with inferiority complex .. and not being accepted elsewhere and nowhere to go, for they fear even losing the one they are in.
Because love is a powerful force and sometimes it gets the better of us in a way that can not always be explained.