Why is tobacco addictive?

EASY ANSWER Basically, nicotine makes a person feel happy (high) but it doesn't last for very long so the person is baited to take it more often to get that feeling. But over time, the person becomes dependent on nicotine to make them feel that way - if they don't take it they'll feel like crap (eg bad headaches)

SLIGHTLY LESS EASY ANSWER

Nicotine's structure is similar to that of cocaine and heroin - in fact, they belong to the same family! They belong to the Alakoids, which are a group of naturally occurring chemical compounds which mostly contain nitrogen atoms. Like its similar compounds, Nicotine works in the brain by stimulates nicotinic acetylcholine receptors in the brain where the firing receptors cause the release of dopamine, a neurotransmitter associated with pleasure and addiction.

Nicotine also influences other brain molecules that control mood, energy level, and memory. Furthermore, nicotine increases the number of its own receptors. It becomes difficult to quit if the brain grows used to the presence of nicotine because of the decrease of dopamine which in acute cases could cause extreme headaches. Smoking also allows for nicotine to be absorbed by the lungs which speeds it to the brain rapidly (7-10 seconds in total) and in large quantities.85-90% of nicotine in blood is rapidly metabolized by the liver and excreted from the kidney with a half life of 2 hours so addicts are forced to constantly receive doses. This forces addicts to take another 'hit', which they can do out in the open because it is legal to smoke.