Why Nicotine Is Addictive

An unknown component of tobacco smoke appears to destroy an important brain enzyme, monoamine oxidase.

The enzyme is vital for breaking down excess amounts of dopamine, a neurotransmitter that triggers pleasure-seeking behavior. Smokers have abnormally high levels of dopamine, which can encourage the smoker to seek the pleasure of more tobacco smoke.

Because smokers are continually smoking, it does not usually create the pleasure feelings by smoking after awhile, it simply seems to make a smoker feel "normal" because they are used to the high levels of dopamine.