We're goal-oriented. If something is easy to get, then we don't want it anymore. If something is hard to get, it makes us want it more. If someone is treating you shitty, you'll try as hard as you can to make them like you.

As much as I hate to admit it, it helps to be kind of a jerk to your girlfriend sometimes, at least at first, if you actually like her. Girls will try and control you, and if they're too successful, they'll lose respect for you. It's part of the default female head-game programming. :-\