it has something to do with your nervious system. Have someone punch you really hard in the arm and just start laughing and see if the pain goes away faster then if you dont laugh. When ever me and my best bud wrestel and try in kill each other (not really kill) he usually wins because he has to brothers in the marines and air force and another half brother in the UFC and they teach him alot. So thats why he usually wins.. Anyway when he like really hurts me i just start laughing and the pain seems to go away quicker