It's also possible that early experiences with tickling triggered a neural connection or association that wasn't there before. I was 4 when I really started seeking it out and I would tickle classmates into the third grade (girls and boys, until one of the boys said it was gay, and I didn't know what gay was but I knew it was really wrong and nothing you wanted to be, so just girls after that, until everyone seemed to get really uncomfortable about it).
Whether I've really identified bi since or just became a lot more comfortable with that part of myself is off topic, but tickling became a thing for me when my male teenage cousins held me down and tickled me for what seemed like forever, and I was ... that was it, man. Just the sound of the word, and someone else mentioned cartoons, and seeing the word in print, I couldn't look away.
So ... body structure and neurochemistry? Almost certainly. Genetics, maybe, maybe not. Ticklish parents know the pleasure it gives and tickle their children, so there's early associations with love and closeness. Fetish? Well, in that I can remember the room, I can remember how it smelled, what it looked like, I remember being held upside down and tickled here to Tuesday, I'd hazard a suggestion that as people suggest people form foot fetishes in childhood crawling near their mother's feet, this could also be a short-circuit thing as well.
P.S. -- Yeah, it was potentially an abusive situation, but for all the fun I've had with it in my life I'm inclined to forgive. I'm just really conscientious about consent when I'm the ler is all. 😉