Why is tobacco habit forming?

Action in brain: Nicotine causes activation of senstive areas of the brain which then releases Dopamine , thus causing a pleasureable effect and reinforcer of a action. This is basis of addiction maintenance of a hedonic ( feel good) state. Other data shows that genes may be passed on that confer increased susceptibilty to its action.

Nicotine: Most of tobacco dependency happens because nicotine is a dependency-producing chemical, causing tolerance (the same amount does less) and withdrawal when stopped. It is also habit-forming because someone smoking 20 cigarettes a day is taking over 400 puffs per day - 400 "fixes" of nicotine. Anything done that regularly, that often, is reinforcing psychologically, as well as physically.