You tickle 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 it. You can't tickle yourself because you can't surprise yourself.
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.
(source)
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