Laughter(watch this) is an outward expression of amusement (and at times, other emotions). It may ensue (as a physiological reaction) from jokes, tickling and others. At rare times, pain can sometimes be used to create laughter, though not necessarily with sadistic or masochistic reasoning.
Laughter is a part of human behaviour regulated by the brain. It helps humans clarify their intentions in social interaction and provides an emotional context to our conversations. Laughter is used as a signal for being part of a group — it signals acceptance and positive interactions. Laughter is sometimes seemingly contagious and the laughter of one person can itself provoke laughter from others. (more)