Smoking has two major components.  One, the most obvious, is the addiction to nicotine, a drug that has both stimulant and relaxant properties.  A drug that is commonly compared to heroin and cocaine with respect to its addiction potential.  The cigarette has been developed into a scientifically engineered, nicotine delivery system - its that plain and system.  The way to keep the user is to make those high's even higher and the ease with which nicotine can pass through the normal "blood-brain barrier" even greater.  That's what makes an nicotinic addiction stick. When a person stops smoking, the withdrawal from this drug, may begin in minutes for some.  On average, the withdrawal symptoms, as painful and uncomfortable as they usually are, will subside after 48 to 72 hours. 

The second component is the habit, the behaviour and a learned one at that.  Every smoker had to learn the behaviour, had to learn how to inhale, how to hold the cigarette and eventually, all of this became second nature.  It became automated, automatic, relegated to the lower brain, soon to become a behavior that required no conscious efforts, no thinking.   Twenty times per cigarette, twenty-five cigarettes per day, 175 times each week, 750 times each month, 9000 times each year.  That's the amount of practice a smoker gets when taking that cigarette to his mouth, inhaling, holding it and exhaling.  Do anything with that number of consistent repetitions and you are bound to get good at it and soon be able to do "it" without thinking.  Of course, that's what many athletes work to create - that automatic, behaviour.  As a smoker, the same "expert" behaviour has been achieved.  So, its no surprise that unlearning this behaviour should be a major focus of any quit smoking program.