The main reason that made me really hate smoking was thinking healthwise ... In the weeks I smoked heavily , I could get tired from doing stuff I normally do like jogging , swimming , going upstairs , etc. and I wasn't even 20 !
From my little experience , if you're serious about quiting , you shouldn't try smoking less cigs per day , or every other miserable "gradually quiting" plans you hear from your buddies ... It's either done once and for all , or never.