My main reason for quitting is because I keep gaining so much weight and did not have the energy or the fitness levels to exercise properly to lose it.
Three years ago I lost 30lbs and kept it off for nearly a year. I was smoking a disgusting amount of cigarettes at that time and developed asthma. I couldn't smoke and exercise so I gave up exercise (ridiculous I know)
So two years later and 40lbs heavier I really have to quit for good this time. Before I could run on the running machine for 2 minutes now I can run for 20 minutes (and it's only two days later). I just want to get fit/healthy and slim for the summer. I spent most of Easter hiding in my apartment chain-smoking as I felt too depressed/self-conscious because of my weight to go out and enjoy the sunshine.
Good luck to everyone. Hope we can all quit.