We laugh when tickled because tickling activates specific areas of the brain that process touch and pleasurable feelings, particularly a part called the hypothalamus. This brain region is involved in both the sensation of light touch and expectations of pain, suggesting laughter acts as a primitive defense mechanism signaling submission to an aggressor to diffuse tension and avoid harm. Tickling typically happens in sensitive areas like the underarms, throat, and feet, which could have evolved to protect vulnerable parts of the body. The laughter produced is not just from finding something funny but may be a reflexive response to perceived distress or vulnerability, a way to communicate non-aggression and social bonding. The brain differentiates tickling by others versus self-tickling, as the cerebellum cancels the tickle response when you try to tickle yourself due to predictability. Overall, laughter from tickling is an involuntary, evolutionarily conserved reaction with components of pleasure, defense, and social signaling.
