Tobacco contains Harmaline which is an MAOI - it both increases the bioavailability of nicotine as well as adding an additional psychoactive effect; stimulant + anti-depressant
I've recently started adding free base harmaline to my vape juice to create a proper tobacco substitute... I found that nicotine alone didn't provide the relaxation that I would get from smoking tobacco. After a fair amount of research I discovered that it was that lack of harmala compounds.