Even if you know you’re about to be tickled, you don’t necessarily know where, so you react by being ticklish. If you try to tickle yourself, it won’t work because your brains already knows that you are going to do it. So You can’t tickle yourself because you can’t surprise yourself.
Yes, You Can tickle yourself. The only place where you can tickle yourself is the top of your mouth. You get tickled mostly by surprise.
Possibly because the roof of your mouth isn’t used to being touched like that, it still tickles if you touch the roof of your mouth.