It's a strange but universally accepted phenomenon that you just can't tickle yourself. Even with the most sensitive spot on your body, be it the back of the knee or the neck, if the touch is our own, the ticklish sensations never seem to come.
A large part of feeling ticklish comes from being caught off guard with touch. The surprise sends your brain into a state of unease and even panic. Strangely, this elicits uncontrollable laughter as a response. More unexpected touching can elicit more intense the ticklish sensations.
Given that the element of surprise plays such a large role in us feeling ticklish, it's no wonder why we can't elicit the same fits of laughter by tickling ourselves. Because our brain can already anticipate the answers to the when-, what-, and how- questions of our touch, it will prepare the body to receive the touch as ordinary. Thus, no panic and no ticklish sensations.