if the guy is average, or not so hot, men assume the girl is not even remotely interested in the guy. same goes with height, and with good reason, many women want men taller than her...dont ask me why...I know women who are 5'2 to 5'6 and still expect men who are like 6'6
women have a reputation for expecting the moon, so if a guy feels he is not in the ballpark, he'll back off.
This is a monster myth that I would like to dispel. Men are NOT shy around good looking women, they are AFRAID of them. Hot girls are one hundred times more likely to shoot you down or cause you misery than women that don't have so many options. Why shove your hand into the fire to get a gold piece, when you can look around and find enough silver to take care of you? Good looking women should not care if men don't approach them. The ones that do are a glutton for punishment or they are worth of their attention. Either way, they won't be going without for long.