Let’s build a human. Let’s give them a complex nervous system, a powerful brain, and upright mobility. Let’s encourage estensive communicative abilities, give them the opportunity to increase their muscle strength, and throw in some opposable thumbs. Alright, so we’ve got a human.
No, wait, let’s make them ticklish.
I can see it now: the powers-that-be, organizing a meeting to discuss the development of the humans, and everyone’s throwing out good ideas, and the creativity is flowing and people are getting excited, sketching out this masterpiece. A four-chambered heart! Eyelids! Let’s use cartilage to save some money on bone! How about we use a ribcage to protect the vital organs? Genius!
Just before finalizing the layout, some fool in the back who hasn’t said anything all day blurts out passionately, “I wanna make them ticklish!”
And everyone looks at him, confused, for they’ve never heard of this adjective before.
“It means I want them to have parts of their body that, when touched, will render them useless, overwhelming them with laughter and watery eyes, and it will sometimes be so severe they’ll have trouble breathing!”
And the others, they must’ve felt bad for this guy, since everyone else had contributed something. “But why?” asked the one who came up with eardrums. “Why give them a weakness?”
“It’s not a weakness,” said tickle guy. “Everyone loves to laugh.”
This they could not argue. The one who developed laughter shrugged and said, “Maybe he has a point.”
“Which parts of the body should be ticklish?” they asked.
Tickle guy said, “Definitely the neck, and under the arms, and behind the knees. I guess the whole knee, actually. Maybe the belly, too. I guess the bottoms of the feet would work. And maybe anywhere that’s touched with the slightest graze. In fact, how about we make the whole body ticklish, but have the intensity of the areas vary from person to person?”
“Seems like the humans might get annoyed with this vulnerability,” said the one who thought of the brain. “It could render them incapable of critical thinking.”
“No way. They’ll love it. They’ll be laughing all the time.”
So it was. Humans were made ticklish, and this is the best explanation I could come up with. Because seriously, why we have parts of our body that evoke oftentimes painful spasms of unwarranted laughter, I have no idea. Oh well. At least we laugh when we’re tickled, and I’ll never argue against a good laugh, although I question said laughter if it feels like torture.