Final answer:
Nicotine's highly addictive potential is due to its effects on the brain's reward system, including increasing dopamine release and activating the reward center.
Step-by-step explanation:
Nicotine, the active chemical constituent in tobacco, exerts its effects through the agonism of nicotinic acetylcholine receptor. This results in multiple downstream effects, such as increasing the activity of dopaminergic neurons in the midbrain reward system and decreasing the expression of monoamine oxidase in the brain. The increase in dopamine release and the activation of the brain's reward system contribute to nicotine's highly addictive potential.