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.