If done gently, it can be very pleasant, but if it goes on for too long or the intensity is excessive, it may become very uncomfortable; so tickling is a classic of playful love games and also an element of torture. In reality, the way your body reacts to tickling is a nothing more than a defence mechanism, a primitive instinct that induces your body into reacting when posed with a situation of danger, ones to avoid, for example, the attacks of a poisonous spider or a scorpion that crawls over your skin.
From asphyxia to a heart attack:
The signal produced by the stimulation of the dermis tactile receptors travels to two cerebral regions: the somatic-sensory that processes touch and the pre cingulated cortex that manages all of the pleasant information. The neurons of these two areas spark an immediate answer in the form of sudden movements of the body and a nervous laughter which is very hard to control. If those reactions interfere with the functioning of the involuntary muscles that control breathing, they may cause spasms or asphyxia. Stress provoked by tickling can also cause a heart attack, but only on people that suffer from some sort of heart disease. The actual documented clinical cases of both situations are rare; usually the most serious consequences of a good tickling are nothing else but the annoyance of the victim while they fan around laughing uncontrollably.

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