Addiction to nicotine is characterized by compulsive drug seeking and abuse, even in the face of negative health consequences. Cigarette smoking produces a rapid distribution of nicotine to the brain, with drug levels peaking within 10 seconds of inhalation.
However, the acute chemical effects of nicotine dissipate quickly, as do the associated feelings of reward, which causes the smoker to continue dosing (inhaling) to maintain the drug’s pleasurable effects and prevent withdrawal. And it's precisely because of it's short duration of action that nicotine is considered more addictive (or at least as addictive) than heroin.
Tobacco Labelling Resource Center