I started smoking when I was 14, and quit 5 months ago (at age 21). I haven't had so much as a drag since July 20th. I know some people say that occasionally smoking (a drag or a whole cigarette) doesn't count as "cheating", and doesn't mean you have to "start over"... But I know if I allowed myself to give in to the cravings and take a drag, I'd be right back where I was before. Quitting is too damn hard to keep doing it over and over again.
I quit because my partner has respiratory problems, and tends to get sick from smoke, dust, or other allergens. Also, I really notice scents, and I hated always smelling like cigarette smoke instead of my perfume or soap.