One reason is the 'hit' that the smoker gets when they inhale. This is brought on by an intense surge of nicotine. Once you have inhaled it takes only about seven seconds for the nicotine from the cigarette smoke to reach you brain via the bloodstream. This is what gives the user that pleasant, satisfied feeling that makes smoking so compelling. Once the nicotine levels in the blood have peaked they rapidly decline until the next puff and then the pattern is repeated. Smoking promotes an intense response as nicotine is quickly delivered to the brain and this leads to an addiction. As time goes on your body comes to expect these nicotine 'hits' that are associated with the smoking habit.