Why do we laugh when somebody tickles us?

I think it is a natural body reaction. Though if you try to prepare yourself or "train" yourself, it is possible to not laugh when you are tickled. And some people will just have a "natural body resistance" to this.:P

i really don't know, i'm tickle-tolerant?.. but i found this info at wikipedia.org .. Self-tickle Knismesis may in fact represent a vestige of the primitive grooming response, in effect; knismesis serves as a “non-self detector” and protects the subject against foreign objects. Perhaps due to the importance of knismesis in protection, this type of tickle is not dependent on the element of surprise and it is possible for one to induce self-knismesis, by light touching. Gargalesis, on the other hand, produces an odd phenomenon, when a person touches “ticklish” body parts on their own bodies, most people measure no tickling sensation. It is thought that the tickling requires a certain amount of surprise, and because tickling one’s self produces no unexpected motion on the skin, the response is not activated. A recent analysis of the “self-tickle” response has been addressed using MRI technology. Blakemore and colleagues have investigated how the brain distinguishes between sensations we create for ourselves and sensations others create for us. When the subjects used a joystick to control a "tickling robot", they could not make themselves laugh. This suggested that when a person tries to tickle him- or herself, the cerebellum sends to the somatosensory cortex precise information on the position of the tickling target and therefore what sensation to expect. Apparently an unknown cortical mechanism then decreases or inhibits the tickling sensation. A small percentage of people however, have found it possible to tickle themselves.

Nice question! Well, maybe simply because it just so ticklish that we can't help but laugh and scream. Maybe because that's how human do react in the first place if someone tickles the tickling part of the body. :)