For me it was childhood flirtation that caused me to have this fetish I believe. I was IN LOVE with this girl who lived across the street. Her name was Erica. I was 8 or 9 and she was 9 or 10 when they moved into the neighborhood. My younger brother and her younger sister were the same age. My other neighbors all had kids around our age too, and one day we were playing wiffleball, which turned into duck, duck, goose. Erica turned around and said "let's make this more interesting, if you get picked and catch the other person you get to tickle them until they knock you off." Of course I picked my secret crush and I let her catch me. I thought she was just gonna be silly but next thing I knew she had jumped on top of me and poked my sides so relentlessly that I almost threw her into a rock when I threw her off. So it was my turn, and I don't remember who I called on, but I remember seeing Erica upset, as if she thought I didn't think it was cute. So when she got picked again and it was her turn to be the one chased, she of course picked me. I tickled her for a while. I couldn't tell if she wanted to get me to stop, or if she was just fake fighting it. To this day, I remember that day and all the rest of the days where we tickled each other for fun and just laughed. She was adorable. But from that first day on I've always loved it.
Maybe Erica is here... who knows ?