I'm not a girl, so forgive me for posting.
But I want to go off of what Bohe said:
Last summer I took a humor in literature course online. So, the first week or so, we studied a few joke analysis theories. One -- Freud I think -- basically said something very similar, though not in relation to tickling. That jokes evolve as we grow older, from play to more obscene things.
The point is that, if that is true, then it also makes sense for that to follow in other areas of life. So tickling as play as a child, yeah, it kind of makes sense that it would develop that way. Hm.
My post was not as interesting as I intended.
Edit:
I had to look it up. The course was still active, so I grabbed the relevant info:
1. Play - production of nonsense. This would be very young children simply playing with language and words, without regard for the meaning. For example, children often invent languages. My daughter invented one called "Utian" and she would produce a few sentences or translations. Obviously, "Ut" (or is it "Uttt"?) was a major vocabulary item, and could be used as an interjection, noun, verb, or adjective. Obviously, this is just nonsense without story or plot.
2. Jest - nonsense produced in the face of criticism. Example: “’Twas brillig and the slithy toves did gyre and gimble in the wabe/ All mimsy were the borogoves, and the mome raths, outgrabe.” Even though this – the opening of Lewis Carroll’s “Jabberwocky” -- is nonsense, the structure and sound give us pleasure and help shield us from criticism. We are still doing basically what my daughter did with Utian, but with syntax and even a plot. The sense that the poem seems to make allows us to indulge in the pleasure of playing with sound and nonsense while still seeming normal.
3. Innocent joke. Nonsense protected from listener’s ridicule by making sense. Example: knock-knock jokes, why-did-the-chicken-cross-the-road jokes, etc. For example: “What has four wheels and flies?” “A garbage truck.” These jokes all make use of sound play, faulty thinking, etc., and recovering the familiar, mostly in the fact that they come in series. However, they do not engage repressed material. They are popular in elementary school through puberty, but give way to tendentious jokes among teenagers and adults.
4. Tendentious joke. The first three stages are still present in the tendentious joke, offering pleasure which allows criticism of the object of the joke to take place, releasing greater pleasure as repression is overcome. The tendentious joke may be set up like a joke at stage #3, but the material it treats or “message” it gives would be subject to social sanction if expressed in a straightforward, non-humorous way. Political jokes, jokes about religion, and sexual jokes are three types that belong in this category.
-Etc
