In addition to Bill's suggestion, which is a good one, perhaps look up trauma bond. This is exactly what the trauma bond does to someone. Through cycles of intermittent abuse and periodic positive reinforcement, the victim in their brain becomes "addicted" to wanting the good times again, and virtually will do anything to have them back. Abuse victims therefore have a very hard time leaving their abuser for just this reason. Also, the abuser brainwashes the victim into thinking it's their own fault.