In my experience, everyone's different, male or female. I've known more shy guys than outgoing ones, and about equal numbers of shy and outgoing women. And a lot of it depends on variables, such as time, environment, and relationships. A guy might be really outgoing with friends, but really shy among strangers, or be a bookworm by day and a party animal by night. As to why a specific guy is outgoing, maybe he's trying to impress someone, or feels he has something to prove.
Just some thoughts, and I'm sure it's far more complex than all of that for any given person in any given situation.