I think many people don't realize that the psychological addicition is every bit as strong as the physical one, or at least, it was for my dad when he tried to quit. Smoking is a habit chemically, but also in terms of physical and social action. Smokers enjoy a cigarette after a meal, while driving, and in social situations. Quitting means not only withdrawing from the nicotine, but changing habits of behavior in each of those situations. Smokers will want a cigarette in a bar, or dricing their car often for years after the nicotine is gone from their system, simply because their mind has a powerful association between the act of driving or socializing, and the act of smoking. It takes time to re-associate those actions with non-smoking activities and thoughts instead.