Did you ever though of how the Addictions develop?

A second important dopaminergic system arising in the midbrain is the mesocorticolimbic pathway, which is implicated in the pathogenesis of addictive behaviors including drug reward. Its key components include the midbrain ventral tegmental area (VTA), medial forebrain bundle, and nucleus accumbens. The cholinergic pathway originating in the nucleus basalis of Meynert plays a role in memory function in Alzheimer's disease.

Addictive drugs share the property of increasing dopamine release in the nucleus accumbens. Amphetamine increases intracellular release of dopamine from vesicles and reverses the transport of dopamine through the dopamine transporters. Patients prone to addiction show increased activation of the nucleus accumbens after the administration of amphetamine. Cocaine binds to dopamine transporters and inhibits dopamine reuptake. Ethanol inhibits inhibitory neurons in the VTA, leading to increased dopamine release in the nucleus accumbens. Opioids also disinhibit these dopaminergic neurons by binding to receptors expressed by -aminobutyric acid (GABA)- containing interneurons in the VTA. Nicotine increases dopamine release by activating nicotinic acetylcholine receptors on cell bodies and nerve terminals of dopaminergic VTA neurons. 

Tetrahydrocannabinol, the active ingredient of cannabis, also increases dopamine levels in the nucleus accumbens. The blockade of dopamine in the nucleus accumbens can terminate the rewarding effects of addictive drugs.