Why can't you tickle yourself?

HowStuffWorks explains that people cannot tickle themselves because the brain is aware that the sensation of touch is impending, and it loses the effect of surprise that usually accompanies the sensation of being tickled. The brain's cerebellum is responsible for deciphering whether a touch is familiar or unexpected.

When people laugh in response to being tickled, they are not actually happy nor do they find the sensation funny. According to HowStuffWorks, laughter is brought about as a result of the panic that the body experiences when it is touched suddenly and in sensitive areas, such as the armpits and ribcage. The panic is evolutionarily designed as a reaction to unexpected physical contact with potentially dangerous things outside of the body, such as spiders.