Smoking is an attachment ! Much like the other habits that we pick up, such as drinking, gambling, sleeping (haha, just joking!), etc.
I have seen smokers who tried but unable to quit, also many who just stopped, cold turkey style. Those who could throw the civil and just quit were those who had a strong personal reason to quit. One of my friend quit on he saw his 4-year old son holding a pencil in his mouth mimicking his smoking action, he was shocked to the core and realized he had been showing his son a bad example. Hence he quit and till today, he had never taken any puff !
You see, it's all in the mind.