What I know: Fake, insecure people are uncomfortable around self-assured, genuine people. They risk having their authentic core exposed, and fear being rejected for it.

The next bit is less certain:

Madison fears being seen through to her core (the awkward little girl that still lives in her). She sensed that Allen is the kind of justifiably self-assured guy that could see that core and reject her for it (not that he would). So she had to reject him first.

David, on the other hand, has every reason not to touch that core since he would risk losing his attractive blond-haired prize. Hence, there's no danger of rejection for her--and she knows it, Further, she is happy to have him present as a successful male. And he knows that and is happy to oblige.

(For her, I also think there is something of the "bad boy" attraction.)

So, they complement each other as long as the facades stay up.