I don't think its looks that does it. Its more how you as a female treat him. If you stroke his ego too much then he starts getting cocky. Same for a girl. Stroke her ego too much and she gets cocky. All in all its more of a balance. Maybe its how you might be treating the good looking guy that brings about the reaction.
The difference between a "good" guy and a "nice" guy is that nice guys have no "back" to them..you can be nice to a girl without being walked on. That's what women want, they want to be treated well but not let others or her treat him like sh*t. "good" guys, like myself, am exactly that
Adding to that, have you noticed that when a guy treats a pretty girl too well, she gets bored from all the butt kissing and dumps him? Maybe that's why better looking guys, guys who have a lot to offer, and/or guys who don't kiss the hot girl's butt tend to be able to keep her. . .