Why is it that we can't tickle ourselves?

Answer 1

Although it is very difficult to tickle yourself in a conventional manner, it is possible to tickle yourself. Studies have shown that the sensations that we get from being tickled include some element of surprise. It is hard to surprise yourself with a tickle. That also includes using a mechanical device that tickles you if you are the one activating it. However, if you really feel the need to tickle yourself, try the roof of your mouth. Most people can successfully tickle themselves there.

Answer 2

Babies enjoy water when a parent whorls them around at the surface on their tummies but hate and fear it when on their backs (never do that!). The difference is that on the tummy there is a higher sense of self-control. Likely a person can tickle you with their fingers because you don't have a sense of control (impossible when you tickle yourself). However a feather in a nostril is ticklish no matter whether you or someone else applies it because of the ticklish (defensive) sensation. Nose hairs help exclude ants, etc. because they tickle.