You cannot tickle yourself because your brain predicts the sensation before it happens.

The cerebellum is the part of the brain that tracks your body movements. When you move a muscle, your brain sends out two signals. One signal tells the muscle to move. The second signal, called an efferent copy, tells your sensory system what to expect.This process is called sensory attenuation. It allows the brain to dampen or mute sensations that you cause yourself. This prevents your brain from being overwhelmed by the feeling of your own clothes or your own footsteps. Because you know exactly where your fingers are moving, the brain cancels out the tickle response.In 1998, researcher Sarah-Jayne Blakemore studied this using brain scans at University College London. The scans showed that the area of the brain that processes touch is much less active during self-tickling. The brain only stays alert for touches it did not plan.This system helps humans stay safe by focusing on outside threats. If a person cannot tell the difference between their own touch and an outside touch, their senses become overloaded. Some people with specific brain conditions can tickle themselves because their brains do not send the prediction signal correctly.