Why do guys try to prove their masculinity when girls come around?

It's basically due to the gender divide and rule, that starts from childhood where each have their own world and lifestyle. So many activities also are exclusive to gender like girlish and boyish..

So they are not used to girls amidst or lack experience in approaching a girl appearing all of a sudden.

But it's really not masculinity or machoism , but just pulling up together from casual to conscious state. Also most girls are well dressed while guys are casual..