Training effects. If people are treated well despite their behavior (good looking people) then positive character traits (being nice) are not reinforced throughout their lifetime. For example, if the pretty girl treats some guy like garbage and yet he keeps being nice to her to try to get into her panties, her mean behavior has been rewarded, despite the fact that the boy in question would drop an average-looking girl like a rock if she did the same thing. Hence, the average-looking people are learning better social skills through constant reinforcement of proper etiquette, rather than indiscriminant reinforcement simply because you're pretty.
Ok, so let's turn this around. Girls drool over tall, attractive guys, but wonder why they keep cheating and messing them over, BUT they don't stop pursuing them and having sex with them. Are the tall, attractive boys punished or reinforced by their bad behavior? Neither. Punishment is made irrelevant because there is always another gullible girl waiting and hoping to be with the guy.
Now let's look at an average guy, maybe even unattractive. If he screws up, she may leave. Finding another girlfriend is not as easy, so her reinforcement tactics are more effective (i.e. punishment works) so he is more likely to behave and be trustworthy because finding another girlfriend will take substantially more time and effort than it would for the tall, attractive guy. HOWEVER, although some women and men are abusive, there is a point in a relationship where being alone is more rewarding than being with someone sick in the head. This is what I decided from one of my previous relationships. Another time I dated a very attractive woman, but she screwed around all the time and didn't care if I left her. So my experience has taught me that what I learned about psychology is true and this is very good question!
Advice: learn to be attracted to people that are not so physically attractive, but are very much worthwhile friends. Maybe that guy you were previously putting off as "just friends" material was really the best person you could possibly date! It's tough though. We get BOMBARDED by our media about "What's hot/What's not" and it's hard to see through that brainwashing...