Why Don't You Laugh When You Tickle Yourself?
As noted, we are most likely to laugh at jokes and situations when we are suddenly surprised by a punch line or something that is totally unexpected. This may explain why someone can tickle you into hysterical laughter but, as Aristotle noted, you get no such response by tickling yourself in the same fashion in the identical spot.
Although most young children instinctively giggle when they are tickled, prolonged tickling has been called one of the worst medieval tortures ever devised.
A tickle has been assumed to cause laughter because it is a reflex response and whether you or someone else initiates this stimulus, the information sent through your spinal cord to the brain should be the same. However, for tickling to make you laugh, the brain apparently requires an element of surprise, just like a joke.
Why this is so important remains a mystery and various types of tickling machines have been constructed to explore this.
clipped from: www.mercola.com