Born with it and wired that way, to be sure...but various experiences can indeed hone it without our even realizing it. Throughout my growing up, tickling my feet was my mother's favorite token way to really make me laugh, and each time she ever started with me, I relished it every time. I would usually sprawl out on the floor in the living room while we watched TV...she sat on the sofa. More times than not, I'd have the tops of my bare feet nestled close to where she sat, and it wasn't usually too long before I'd feel her softly skittering her fingernails along my feet. There were no fetish-oriented sensations or conflicts of ideals, I just loved how it felt and how much she could get me to laugh.
As I got older (maybe...13 and beyond), the thoughts of sharing similar experiences with girls my age would foster more excitement than I knew what to do with. I thought I was considerably left of center for it until I discovered my first issue/copy of "LEG SHOW" magazine. Once I read that issue cover-to-cover, I realized immediately that I wasn't alone, or too left of center after all, and it enabled me to be a bit more open and tactfully bolder in my endeavors (by then, I was 17). I never pushed the envelope with anyone, always respected limitations (after all, it isn't for everybody...much as we wish it could be), and some of my greatest experiences were shared with females where mutual trust, respect and a genuine 'like' was established between us.