I do it only to certain girls, because their reaction is funny, but I guess other guys can have their own reasons. My reason is just for entertainment and to make them smile, because usually they're ticklish, and I try to break an awkward silence or stare with it. Other guys could use it to flirt, but I use it as a friendly gesture.

I used to nudge there shoulders and did you know that if you touch someone they are more willing to talk to you, found that out in my interpersonal communications class, no joke, but of course do it in an appropriate place