Watching teen boys interact when I was a high school teacher, I concluded much of the behavior is hard-wired...and that behavior is dominance, which also affects how boys interact with each other. Using intimidation and coercion happens in male relationships, too. The concept of a "gentleman" comes about because it is somewhat unusual for a man to be gentle. It requires definite societal intervention. This isn't to judge boys and men for being "wrong" but to point out that aggression and dominance pay off for the male gender, so when they get to be 13 or 14 it's all about not being "like a girl" which is the ultimate insult. Weak, different, less than in their minds. Not all boys, not all men--but this kind of behavior and posturing goes along with male culture. We've taught them to be gentlemen for a reason, but it seems to start new with each generation.