I was a lanky kid in high school and college. A couple of my friends who were serious lifters took me to lift with them. Took a couple years to figure out what I was doing in the gym and at the dinner table. Once I got the first pump in my workout, I was hooked.

A few reasons that come to mind include:
1) the pump
2) the sense of accomplishment after a good workout
3) the endorphin release after hi-intensity cardio
4) watching my body transform over time