Much to the dismay of wacky masochist everywhere, the human brain is wired against self-tickling. Because the brain controls movement, it knows what your hand is going to do before you do it. Thus it anticipates the exact force, location, and speed of the tickle and uses that information to desensitize you to your own roving hands.

So why do we have a tickle response anyway? Turns out, it’s a defense reaction meant to alert our cave-dwelling ancestors to creepy crawlies that didn’t know their place, and the uncontrollable laughing fit that goes along with it is actually a panic response. Even if you know someone else is about to go for your rib cage, it’s hard to turn the response off because a) your brain can’t anticipate exactly how and where they’ll tickle you and b) knowing someone is about to tickle you is usually enough to keep those panic receptors open and ready to go.

-from mentalfloss.com

See also: Tickle Salon and Tickle Robot Andre Stubbe and Markus Lerner’s ticklish robot for something completely different: trout tickling [wiki], and who can forget: Tickle Me Elmo [wiki]