Chemical rush, everything gets going your simulating and intense level of emotional excitement with your own self doubt. Thats the "drive" part as far as I'm concerned there's allot to the brain chemistry of desire but I think it's part of a greater hole.
Imagine that we had two buttons, a love button and a screw button. Some things push one and not the other we would like to believe that we only have one. I think there are two (at least) separate systems at work. The Madonna /whore and nice guy/jerk aspect of our romantic attraction and behavior is a reflection of that. I know that's not what your asking.
Why does it take "bad" things to get things going I have no idea. You see much more motivation in people when there is doubt about a relationship than when things are good. You hear of so many stories where relationships get to the brink of exploding before people are forced to change.
Maybe that's just a reporting bias. The people who would just change because you asked them politely to "help create a better relationship" just do it and don't think it's a big deal. I hope it's that simple for some people.
What the problem I see is that the excitement through instability kind of "works" as far as keeping things lively in a relationship but also creates an instability in that relationship.
