Laughter causes other people to laugh.
Once a character starts to laugh, all those around him start to laugh as well. Truth In Television, but useful in fiction for getting a group some release from tension. Also, when a proud character is humiliated, this lets the crowd join together against him. A character who does not join in is separate in some way -- this may be good, because the humor is unsuitable, or bad, because he's too proud to see the humor. It can result in an attempt to fake it. Drives the Everybody Laughs Ending.
ExamplesAnime and Manga