Tickling is a rather interesting phenomenon: When humans or apes are touched in certain areas like the armpits or sides, we respond with laughter AND frantic attempts to stop the assault. Obviously our ticklish areas tend to be very vulnerable, so it makes sense that we would want to protect ourselves, but why do we respond with laughter? How might both of these reactions to tickling have evolved? Do they have the same evolutionary purpose or different ones?
Tickling probably evolved from a defense mechanism but then gradually changed into a more social action, as explained in Provine, 2005 (PDF):
The neurological mechanism of tickling probably evolved from a reﬂex defense mechanism that protects our body’s surface from external, moving stimuli, probably predators or parasites. Our response to tickle is more varied and complex than the typical reﬂex, but it has some stereotypic, reﬂex-like properties (i.e., we laugh when tickled, struggle to escape the tickler, huddle, fend off the tickling hand). Although you can be tickled to laughter by a machine (Harris, 1999) (PDF), most everyday tickle is yet another social context for laughter and a form of communication.