Why do guys flirt? He flirts with everybody.?

One of my guy friends flirts with a lot of people. He does it in a way that is so blatant--like ridiculous comments and pick-up lines--that it makes all of his/my friends laugh. Yet when the girls aren't around, everyone laughs about the girls' reactions and he doesn't seem to mind. He is honestly one of the nicest guys I know--joking, very gentlemanly, considerate, and nice--and all the girls like him, but it's one thing that really slightly annoys me about him. He doesn't really do it to his best girl friends--like he flirts (which I don't mind because that's just the way he is) but not in that ridiculous way he does with other girls. Why is he like that?