There are so many external and internal stimulations striking you at that same time that your brain has learned to filter them out.
The first ones that get ignored are ones that you do which is why you probably don’t notice your tongue movements when you chew or why you can’t tickle yourself.
When you are tickled, your laughter is the reaction that occurred because that ticklish feeling sends us into a state of panic. This is a kind of defense mechanism that developed back in our cave-men days – yes, we used to hunt our prey and prey hunted us by tickles 😉
So when you try to tickle yourself you are in control of the situation and thus there is no reaction.
Did you know?
Science has been able to design a robot that allows people to tickle themselves. To use the machine you would have to lie on your back with your eyes closed. The robot, located near you, would have a piece of soft foam attached to a plastic rod which you would control by joystick. You would maneuver the remote control and after a short delay, would respond. This takes advantage of the delay in the cerebellum.
Note that the image above is NOT that invention 🙂