Why is it almost impossible to tickle yourself?

Some of us are more ticklish than others, but nearly everyone is unable to tickle themselves. The answer is tied to how we see and how we perceive movement. To get to the bottom of why we can't tickle ourselves, let's first examine another phenomenon. Close one eye, and then carefully push against the side of your other (open) eye, moving the eyeball from side to side in its socket. It should appear as if the world is moving, even though you know it isn't.