Abusers use -Intermittent Reinforcement. Which means they are nice sometimes and mean sometimes. You stay to get the "reward" of their sometimes pleasant behavior. That is why it is so puzzling.

Definition:

Intermittent Reinforcement - Intermittent Reinforcement is when rules, rewards or personal boundaries are handed out or enforced inconsistently and occasionally. This usually encourages another person to keep pushing until they get what they want from you without changing their own behavior.