Why do you laugh when you get tickle?

I think it is because when you are tickled, there is a certain type of sensation from this action that travels to the upper part of the body, as if it were a vibration of some sort. Then, when it reaches your upper chest area, to help release the "vibration," you laugh. I think that most of the time, when you laugh, it comes in many short outbursts of "HA!"