Nicotine is one of the most addictive drugs available, although I might argue for caffeine.
Smoking is psychologically relaxing, while physiologically it is a stimulant experience. Nicotine acts on the “pleasure/reward centers” of the brain. Dopamine, the brain chemical responsible for driving these “centers”, is increased as nicotine circulates through the brain. Dopamine is also the chemical stimulated by other drugs that give a pleasant brain experience. Cigarettes (tobacco?) also contain an as yet undetermined substance that decreases the brain level of “MAO”, the enzyme that breaks down dopamine. Less breakdown means more dopamine bathing the brain and more “pleasure” (a rationale used in some antidepressants).
Smokers continue to smoke (and can’t quit) to keep their brain dopamine levels high. It is not much of a leap to realize that sometimes, and for some people, the nicotine “pleasure” isn’t quite enough anymore. Wouldn’t/couldn’t that lead to desire for other dopamine stimulators and serve as the “gateway” to other drugs?