Because to keep the image that they are macho ******.
its not just guys many people do so and its because if they have good friends they know they won't be judged for how they act.
They feel more comfortable or have adapted to certain customs or social norms when around certain people
Because we play a part in our group of friends as there are things expected of us. Iv'e noticed I make my friends laugh whenever there's a lull in the conversation. And i act pretty stupid to do it not like me at all but no one else bothers to because they expect me to do it. Its my thang so to speak
Because they are guys and mysterious :)
Sometimes I act a little different with different friends depending on their mood/vibe. It kinda rubs off on me lol