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.
Whenever the urine becomes acidic, the body excretes nicotine at an accelerated rate. Thus, when a smoker encounters a stressful situation he loses nicotine and goes into drug withdrawal. Most smokers feel that when they are nervous or upset cigarettes help calm them down. The calming effect, however, is not relief from the emotional strain of the situation, but actually the effect of replenishing the nicotine supply and ending the withdrawal. It is easy to understand why the smoker without this basic knowledge of stress and its nicotine effect is afraid to give up smoking.