WHY IT IS NEARLY IMPOSSIBLE TO TICKLE YOURSELF

It's very hard to tickle yourself because your brain anticipates things going on around you in order to help speed up response times. More technically, the cerebellum monitors body movements and can also distinguish between expected sensations and unexpected ones, generally resulting in diminishing or completely discarding expected sensations, while paying much more attention to unexpected ones

So your brain is actively anticipating touch sensations. When it is doing this, it is also actively discarding sensation that it deems unimportant, like when you are typing and it significantly dulls the touch sensation in your fingertips so that you don't really notice it unless you consciously think about it. This same type of thing happens when you try to tickle yourself.

So in a nutshell, you can't usually tickle yourself because there is no element of surprise.