Chris Frith is a professor of neuropsychology with the Wellcome Trust Centre for Neuroimaging at University College London, and his book Making Up the Mind: How the Brain Creates Our Mental World explores the the relationship between our physical brains and how we experience the world around us.
Among other things, Frith provides a neurological answer to a question almost every child has asked and most adults (including yours truly) continue to wonder about: Why we can't tickle ourselves? The reason, Frith writes, "lies in prediction."
He explains that our brains dull our awareness of the many times a day when we touch our own bodies, but focus sharply on contact with other people. For example, when you sit at a desk and twiddle your fingers, your brain knows all about it ahead of time and your conscious mind largely tunes it out. But if someone else takes hold of your hand, your brain is surprised and you become acutely aware.
Frith writes, "If I start stroking your hand while you are having your brain scanned, then I can observe a dramatic increase of neural activity...But if you stroke your own palm in just the same way, then I will notice very little increase in activity. When you touch yourself your brain suppresses your response."
The upshot: When we tickle ourselves, our brains see what's coming and decide it's no big deal. But when other people tickle us, our brain is surprised and responds, triggering a strong physical reaction.
Interestingly, though, there is an exception to this: Schizophrenia sufferers who experience a "delusion of control" — a belief that someone else is moving their limbs — can in fact tickle themselves.
"Those who tickle themselves may laugh when they please."
~ German proverb