Naturally people ask that here occasionally. A common pattern is hearing someone say they watched or read something that seemed to "awaken" it in them decisively, even when they were very young.
I think I'd always been fascinated with the early discovery that one's will isn't the only determinant of one's body's actions – that it's possible for anyone's body to be "unfairly" manipulated to feel or do things they can't control or prevent. I wouldn't learn thoroughly about brains or nervous systems for some time, so that was a source of deep fascination. Ticklishness seems to epitomize it.
Predictably, there are a number of other things that play on that idea that I've found similarly fascinating when I eventually encountered them.
This is pretty much what hooks me about tickling. And it suddenly hit me one afternoon when I was nine years old at school and saw a girl that I liked lead a tickle attack on another boy. It was like being struck by lightning. My life took a different course from that moment.
I speculate that if I had not liked that girl so much, I would not have been so interested in what I witnessed. There was something about her. She made me link tickling with power and manipulation, and made me see that that could be sexy. Not that I understood the meaning of that connection at the age of nine. But in retrospect I see that that is what was happening.