by Ansley Kirbo
This question mostly boils down to what is called a play frame. This is when we see not-so-funny things in a hilarious light. Usually this does not happen with more serious events, such as falling off a cliff, or if someone takes a fall and stands back up with a bloody gash across their face. Laughter is thought to be the simple release of energy. Our minds do so much work, laughing is an easy way to get it out. Also, falling happens so unpredictably, that we have an almost immediate and uncontrollable reaction. So that is, basically, the reason we laugh when people fall down.
If you would link to learn more, click this link: Why Do We Laugh When Someone Falls?