The sensation of tickling has been scientifically investigated for more than 100 years (and unscientifically for many centuries before that) There are many unknowns associated with tickling in general. e.g.
Why are some body areas more ticklish than others? Why are some people more ticklish than others? Is there an evolutionary advantage for ticklishness? Does it exist as part of parental 'bonding' process? How is the sensation physiologically generated? How and why does tickling trigger laughter? Why is 'self-tickling' problematic?
Further info. at Wikipedia
Note: Many mammals other than humans are known to be susceptible to tickling - and also some reptiles and fish.