Nicotine acts as a stimulant on the central nervous system. Nicotine has many metabolic affects. It increases the metabolic rate, raises blood pressure, changes muscle tension and affects certain brain chemicals and hormones. These and other metabolic changes create a pleasurable sensation in the smoker that is felt as relaxation. Another factor is that the 'tolerance' to the effects of nicotine develops very quickly.
Tolerance is a word that kind of means the smoker wants to smoke more and needs to smoke more.
You NEED to smoke more to get the same desired effects from the nicotine. Once you become addicted your body depends on the presence of nicotine. Some scientists have stated that the addiction to tobacco is harder to break than the addiction to heroin. Once you try to stop smoking withdrawal symptoms occur. Nicotine is actually a deadly toxin.