Mostly teenage anger and rebellion. I felt my parents were to strict, not letting me decide if I PERSONALLY wanted to be mormon or not. Hadn't had interest or paid hardly any attention in church since i was 14, then when i was 16 I went away to a private school for 9 months, got to see what it was like to not be forced to go, and i liked it.
now, more than a couple years down the road, i've gotten a little bit more interested in religion as a whole :)