From Wikipedia:
By binding to nicotinic acetylcholine receptors, nicotine increases the levels of several neurotransmitters - acting as a sort of "volume control". It is thought that increased levels of dopamine in the reward circuits of the brain are responsible for the euphoria and relaxation and eventual addiction caused by nicotine consumption. Nicotine has a higher affinity for acetylcholine receptors in the brain than those in skeletal muscle, though at toxic doses it can induce contractions and respiratory paralysis. Nicotine's selectivity is thought to be due to a particular amino acid difference on these receptor subtypes.
tobacco smoke contains the monoamine oxidase inhibitors harman, norharman, anabasine, anatabine, and nornicotine. These compounds significantly decrease MAO activity in smokers. MAO enzymes break down monoaminergic neurotransmitters such as dopamine, norepinephrine, and serotonin.
Chronic nicotine exposure via
tobacco smoking up-regulates alpha4beta2* nAChR in cerebellum and brainstem regions but not habenulopeduncular structures. Alpha4beta2 and alpha6beta2 receptors, present in the ventral tegmental area, play a crucial role in mediating the reinforcement effects of nicotine.
It think it says:
Nicotine makes you feel awake, satisfied, and stimulates your happy face.
Tobacco smoke (not vaping)
also makes it so your body chemistry can't stimulate it's own happy face.