on a base physical level, laughter occurs because your brain short circuits. As you read or listen to something your left brain does all the work. The second your Right brain sees a pattern that is contradictory yet a pattern. your brain sends the signal from the right to the left and back so fast your brain literally shorts out and that is laughter. Your brain literally can not figure out which of the two brains is wrong ... If any??
did you know for the last 12 000years mankind has never stopped laughing ... some where all the time somebody is laughing. In fact at any given second over 1 000 000 people are laughing all over the world. There has been a new rage on youtube with the event of the "Try not to laugh" video challenge. this one is one of the best I have seen.