About 10 seconds later, the nicotine reaches the brain, where it locks onto specific receptor areas. When that happens, a chemical called dopamine is released into the brain, which causes the smoker to experience pleasurable, positive feelings. People who smoke have difficulty quitting because they come to depend on the good feeling they get from nicotine and want to experience it repeatedly. This dependence is a sign of addiction . Nicotine is both physically and psychologically addictive.
Physical addiction is biochemical.
A person who tries to quit smoking experiences withdrawal symptoms such as depression because the brain expects nicotine and becomes physically distressed without it. Smokers also tend to associate cigarettes with other pleasurable activities such as relaxing with friends or reading the newspaper at breakfast. This psychological addiction is hard to break because the smoker resists changing these daily routines. Ironically, tobacco-related illnesses are not directly linked to nicotine itself, but result from exposure to the several hundred other chemical substances inhaled in tobacco smoke or absorbed from chewing tobacco or snuff. Some of these chemicals come directly from the tobacco plant, while some are produced in chemical reactions as the cigarette burns.