Tickling is a trauma for some people although some people find tickling enjoyable and not traumatizing.
Tickling can be trauma for some people if the tickling is unwanted or goes on too long.
Tickling someone can trigger a persons fight or flight response and cause the person discomfort, pain or even a sense of feeling helpless.
For some people the tickling can also be associated with some past trauma, which can lead to anxiety or negative reactions even to the mentions of tickling.
Tickling of someone without the persons consent or when they've ask you not to or to stop can be a form of boundary violation and can also be distressing.
The part of the brain that does not allow you to tickle yourself is your cerebellum.
The cerebellum is located at the back of your brain and is the primary brain region which is responsible for preventing you from tickling yourself.
The cerebellum monitors your movements and predicts any sensory consequences of your actions.
And by doing so it cancels out the tickle sensation when you initiate the tickle yourself.
Your brain knows to expect the tickle and so it doesn't allow you to feel the tickle when you've done so yourself.
Basically when you move your own hand to tickle yourself, your cerebellum anticipates the touch of the tickle and the resulting sensation.
The prediction of the tickle allows your brain to suppress the tickle response which makes the tickle sensation less intense or even absence.
Your cerebellum distinguishes between external touch such as when someone else tickles you and the self generated touch is prevented when tickling yourself.
Also the somatosensory cortex that processes touch and other sensory information, is also involved in the tickle response although it's activity is reduced when the cerebellum predicts a touch that is self generated.
The ticklishness is also influenced by factors such as context and mood, with situations of fear or anxiety being able to suppressing the tickle response.