I think as a whole it's society, women are expected to 'fix' bad relationships and damaged men. And nurture.
My own personal reasons, I think were:
Emotionally cold parents who put me down and critisied me continuously. I've never had a compliment from them. But practically they were always there. So my perspective of relationships was the practical matters, my emotions don't.
Very low self esteem and eager to please, due to above.
And as a defence mechanism, I somehow became a 'fixer' I used to be convinced I could love anyone better no matter how vile or useless. 
I was just used to it, and thought its ok I can take this, I'm strong I just get on with things.
Secretly I had an awful fear of abandonment and to be honest, deep down I knew I was better than them, so that gave me the comfort of thinking they would never leave.
Fucked up I know!
After counsilling and a period of singledom, focusing on myself and raising the bar and realising my self worth,hopefully I've broken this pattern.