One truly unique smoker once told me she smoked to breathe better. Another once said she returned to smoking when experiencing chest pains. She figured the fear of a heart attack is enough to make anyone smoke. None of these reasons satisfactorily explain why people continue smoking. However, the answer is, in fact, quite simple.
Smokers smoke cigarettes because they are smokers.
In the cigarette smoker's case, nicotine is the culprit. He is at the point where the failure to maintain a minimum level of nicotine in his blood stream leads to the nicotine abstinence syndrome, otherwise known as drug withdrawal. Anything that makes him lose nicotine makes him smoke. This concept explains why so many smokers feel they smoke under stress. Stress has a physiological effect on the body which makes the urine acidic.