Our sense of smell is closely tied to memory, and so it's only natural that smelling someone smoking will trigger a yearning in a former smoker [source: New York Times]. Smelling fresh smoke will evoke memories of cigarettes past, moments enjoyed. Watching someone smoke combines the triggers of seeing and smelling it, which can be nearly unbearable for some quitters. That's why people who successfully quit often ask for smokers among their friends and family to avoid smoking around them.
To a nonsmoker or someone who dislikes smoking, the smell is acrid. Smelling smoke in a closed space, on clothing, in a car, even on pets can induce strong negative reactions. But for a former smoker, often the scent has the opposite effect. The smell can be almost comforting and certainly suggestive. It is also reminiscent of how your own clothes, hair, car and the like, used to smell. Sometimes this works as a positive reinforcement. If a former smoker comes to dislike the stale odor, then catching a whiff will just remind them that they're glad they quit.