Substance Abuse/Dependence (technical term for addiction) is not just a habit which you can quit anytime (and usually restart again very soon) but it is a proper disease with biological underpinnings i.e. Even a single puff of cigarette or a sip of alcohol brings about changes in the brain which can lead to addiction. These substances release a chemical in the brain called Dopamine which gives us pleasure and happiness which makes us get hooked to them. However over time we require higher amount of the substance to get the same amount of high which we got initially with a very small dose.This happens because of 'neuroplasticity' and 'tolerance.These higher doses cause mental and physical harm but person continues to do it because by that time the mind and body is totally addicted to it and now not taking the substance causes pain or restlessness or lack of pleasure or any other withdrawal symptom.But people continue to think its just a habit and i can kick it myself till the very late stage when it becomes unbearable or very dangerous for the health!