Because your brain is always predicting your own actions, and how your body will feel as a result, you cannot tickle yourself. Other people can tickle you because they can surprise you. You can’t predict what their tickling actions will be.
And this knowledge leads to an interesting truth: if you build a machine that allows you to move a feather, but the feather moves only after a delay of a second, then you can tickle your- self. The results of your own actions will now surprise you.