I started smoking when I was 13. Mostly because it seemed like something "forbidden" to do. My parents smoked, so it was easy to get access to cigarettes.
As I got a little older, the "cool kids" smoked, so it gave a shy kid some openings for socialization.
I quit 5 years ago, and it was easily the hardest thing I've ever done. Some mornings I still wake up and reach for my pack before I'm fully awake