If you've ever found yourself having difficulty extricating yourself from an abusive relationship, or if you have a loved one who is trapped in one, I strongly encourage you to read this article. It's a bit lengthy, but worth every minute you devote to it.
Dr. Joseph M Carver, PhD answers this question better than I've ever been able to do so. Please see:
The Mystery of Loving an Abuser, part 1
The Mystery of Loving an Abuser, part 2