The irresistibly of others' laughte

The irresistibly of others' laughter has its roots in the neurological mechanism of laugh detection.
The fact that laughter is contagious raises the intriguing possibility that humans have an auditory laugh detector -- a neural circuit in the brain that responds exclusively to laughter. (Contagious yawning may involve a similar process in the visual domain.)
Once triggered, the laugh detector activates a laugh generator, a neural circuit that causes us in turn to produce laughter.