When someone tickles you under your arm, or on the soles of your feet, you start laughing uncontrollably. You are tickled mostly because of surprise. Even if you know you're about to be tickled, you don't necessarily know where, so you react by being ticklish. When you try to tickle yourself, it usually doesn't work, because your brain already knows how you're going to do. To put it simply, you can't tickle yourself because you can't surprise yourself!
The feeling you have when you are tickled is actually similar to what you feel when bugs or spiders crawl on you. You reaction to tickling is actually the brain's reaction to an unexpected sensation, whether it is a tickle, or the crawly feeling of an insect on your body.
Amazing : Do you know, People who can tickle themselves are high talented and have power of understanding things fast... Lol! let me tickle now and check..