Addiction has to start with exposure, says Dr Gillian Tober, president of the Society for the Study of Addiction. "It's generally for social reasons – groups of friends or a boyfriend or girlfriend – and it's often not pleasant. The reward is merely social. It then becomes reinforced and casual use shifts to dependence."
Drugs directly feed the reward circuitry of the brain, she says, and the brain learns to look forward to the thrill. Tolerance occurs as you demand more each time. Physiological dependence – addiction – then emerges. "It is this area – the mechanisms involved in the addictive process – where research has been most progressive and this has meant we have a lot more effective medications to help people come off nicotine, alcohol and opiates," Ilana Crome, a professor of addiction psychiatry at Keele University, says. "But we still don't know what actually causes addiction."
For more, go to here