My instrument (classical/flamenco guitar) called to me. I grew up liking music, but I had an unsuccessful first start with piano lessons when I was seven years old - a few months of lessons from a neighbor, but I rarely practiced and just didn't love it that much - I even got in trouble for lying about practicing once! I just wasn't mature or self-disciplined enough at that point to discover the rewards of focused practice. I then played the saxophone in middle and high school, but it was a social thing and hardly a serious pursuit. But when I started playing guitar several years later, and then shortly after when I discovered flamenco, I was hooked for life. Now here I am playing guitar four to six hours a day preparing for a concert, teaching several lessons a day, and I can't imagine doing anything else for a living. Interestingly, piano was my second instrument as a music major, and we got along really well the second time around. Sometimes the right instrument finds you at the right time, and the rest of your musical life falls into place around it!
I'd be curious to hear how others discovered their instruments. Did anyone crossed paths with their current instrument earlier in life without a 'spark' and then later discover that it was their calling?