Can someone explain the "Don't touch my girlfriend" thing?

We've all seen this. Guy tells his buddies "Don't touch my girlfriend..."<br><br>And then, one of his buddies pretends to go touch her as if to say "oh I'm really gonna touch her..."<br><br>And the guy gets all angry and protective...<br><br>Can someone tell me why that is? I don't get why they do it. The other guy is just joking. What does it even mean? It looks so stupid.