Laughter is an instinctive, contagious, stereotyped, unconsciously controlled, social play vocalization that is unusual in solitary settings. Laughter punctuates speech and is not typically humor related, speakers often laugh more often than their audience, and male speakers are the best laugh getters. Laughter evolved from the labored breathing of physical play, with the characteristic “pant-pant” laugh of chimpanzees and derivative “ha-ha” of humans signaling (“ritualizing”) its rowdy origin. Laughter reveals that breath control is why humans can speak and chimpanzees cannot. The evolution of bipedality in human ancestors freed the thorax of its support role in quadrupedal locomotion, a critical step in uncoupling breathing from running, providing humans with the flexible breath control necessary for speech and our characteristic laugh. Tickle, an ancient laughter stimulus, is a means of communication between preverbal infants and mothers, and between friends, family, and lovers. Because you cannot tickle yourself, tickle involves a neurological self/nonself discrimination, providing the most primitive social scenario.
Tickling is often fun for the tickler (the person doing the tickling) and can easily be fun for the ticklee too (the person being tickled). It revolves around touch, which is another reason we feel better after we're tickled. Tickling can be a great (and possibly regular) bonding experience between friends, lovers, family members etc.
