Because if you don't laugh, you will cry. Loud, big huge sobbing type of crying. Or just lose your darn mind. Or both. This is what happens in my house when you take 10 minutes to get the five year old twins ready for the bus and are not watching the two year old like a hawk.
Yes, my two year old did this. All on his own. He's lucky he is cute.
Do your kids ever do things that are so outrageous that you resort to giggles as a coping mechanism?