Tickle laughter is different from other laughter
Let's start by doing what any scientist worth their salt would do: Hook some willing participants up to fMRI machines and make them listen to people getting tickled.
What researchers who did just that found is that when tickled people laugh, it's an entirely different function than "social laughter," the noises you make when you find something funny, or even "taunting laughter," which is apparently a thing that needs studying.
For one, tickling laughter sounds different -- it has a "higher acoustic complexity" -- and activates parts of the brain that process auditory information, as well as a part of the brain related to language comprehension. In short, tickling laughter has meaning. But what?
Well, the researchers describe tickling laughter as an "unequivocal and reflex-like social bonding signal," related in part to how your brain processes language and working memory. It helps you form bonds with those close enough to tickle you, and your response signals that you have a relationship with the tickler. Social laughter grew out of this basic bonding response, but it can appear in scenarios that have nothing to do with touch; we humans have evolved to laugh at movies, for example, while chimps remain totally ignorant of Hollywood.