Why is smoking addictive?

Dopamine. Dopamine is the main pleasure chemical in the brain, basically all drugs that can be addictive raise Dopamine levels in the brain including nicotine. Dopamine levels go up, feel good, levels go down feel bad. With addiction it takes more drug such as nicotine to raise levels with time and levels of Dopamine run out quicker. Thus more drug is needed, more frequently to get the Dopamine boost.
Nicotine addiction. Addiction to smoking occurs because of a substance in tobacco called nicotine, which is absorbed in the bloodstream as smoke passes through the lungs by inhalation. As a psychoactive drug, nicotine acts as a stimulant that causes the liver to release glucose, and the brain's adrenal medulla to release adrenaline. Smoking makes them feel relaxed and alert at the same time, and feel euphoria.