What is the mechanism by which people get addicted to cigarettes?

By smoking them: Part of the addiction is chemical. Nicotine & other ingredients affect the body's demand for more. There is also the actual "good for you" benefit of the breathing pattern associated w/ smoking. A deep inhale, holding, & full exhale. I counsel people quitting to continue this breathing pattern whenever they want - just without the cigarette. That way their body still benefits from relaxing breaths.