INFO:

Areas that are more prone to getting tickled are our vulnerable areas like neck, stomach and armpits. Some researchers suggest that laughing on getting tickled by others could be a defense mechanism. It has evolved over the years when touched suddenly in vulnerable areas. This laughter could either be a way to laugh out of a tense situation or to show submission to the person tickling you to stop further tickles. Now, when we tickle ourselves, our brain is programmed in such a way that it predicts and recognizes our own movements. It knows where our hands can reach. There is no element of surprise or threat, even if touched in vulnerable areas. Hence, we can't tickle ourselves.