We don't know how these are mediated biologically but there are some interesting concepts worth noting.

Alcohol

Cocaine, Amphetamines, Stimulants

Nicotine

The above are potentially addicting.  One interesting theory is that Dopamine pleasure circuits are ultimately stimulated regardless of the agent used.  When the stimulation ends, the euphoria wears off and the user craves to have the circuit stoked again.

Treatments vary depending on the agent used.

Sometimes the treatment aims to replace a pulse of dopamine, a spritz if you will, with a steady stream of stimulation.  Then the steady stream is decreased gradually to ease the withdrawal and craving.

You puff your cigarette.  Inside a chain reaction is set up.  Nicotine hits acetylcholine receptors and this triggers a spritz of Dopamine in your brain that ends quickly.  This brief pulse ends, and your brain cries, "Give me another".

Take Nicotine patches and now you get a steady stream of Dopamine activity all the time.  Then you gradually reduce the dose on the patch until you can be weaned off nicotine completely.

Zyban (Wellbutrin) may help smokers the same way.  It is a Norepinephrine/Dopamine Reuptake inhibitor.

More to come.