Today's interesting fact
So, if someone else's touch can tickle us, why can't we tickle ourselves? Much of the explanation for this question is still unknown, but research has shown that the brain is trained to know what to feel when a person moves or performs any function. We aren't aware of a lot of the sensations generated by our movements. For example, you probably don't pay much attention to your vocal cords when you speak. For the same reason, we can't tickle ourselves. If we grab our sides in an attempt to tickle ourselves, our brain anticipates this contact from the hands and prepares itself for it. By taking away the feeling of unease and panic, the body no longer responds the same as it would if someone else were to tickle us.
Howstuffworks | Why can't you tickle yourself?