Laughter is contagious!

Has someone tried to tell you a story, or a joke, but couldn't get to the punchline because they are laughing too hard?? CD tried to tell me about a conversation he was involved in at lunch...took him ten minutes to get to the end of the story and I ended up laughing at him laughing.. If you know CD, you know there is not much that will crack him up like an idiot, but this did. It was really not a funny statement, but something about the way the guy said it almost sent him to the funny farm.

Why do we laugh when others are out of control with laughter? It is almost impossible for me to keep a straight face when someone is laughing so hard.