Why… doesn’t it tickle when you tickle yourself?
Scientists aren’t completely sure what makes someone ticklish. One theory is that being ticklish is a defence mechanism that protects parts of your body. Another theory is that tickling encourages social bonding with other people. What scientists do know is that being tickled stimulates a part of your brain called the hypothalamus, which is the area that controls emotions. This causes your body to have an emotional response, such as laughing out loud.
The reason you can’t tickle yourself is because your brain knows it is going to happen, so it cancels the response your body might normally have when another person tickles you.