Disconnecting

Why is it so hard to disconnect and let go of people who have hurt you and caused you tons of pain? What is so special about this person that we just cant move on and let them go? I guess its because we still expect something to happen with them. Maybe we think they might change and the situation may work out. We know thats probably not the case but we hope so deeply that it would. This is one obstacle that I can't over come right now. Its just to complicated and I hate it.