Your Brain Cancels It

You can't tickle yourself because your cerebellum predicts the exact feeling your own movement will create, then quietly subtracts it before you notice. When the prediction matches reality, the brain cancels the signal. A real tickle needs surprise that only another person, or a cleverly timed machine, can deliver.

That prediction trick exists for a good reason. Without it, every scratch, every shift in your chair, every brush of your own hand would hit your brain as a fresh, possibly dangerous event. Filtering out your own touch lets you notice the one signal that matters — something outside you.