What does drug and alcohol dependency have in common with Type 2 diabetes, heart disease and cancer? They are all chronic illnesses, with roots based in genetic and environmental causes. Substance dependency is not a question of willpower. Just like you don't choose to have cancer, you don't choose to be an addict.
A person with a substance dependency will keep using drugs, no matter the consequence, because of chemical changes in the part of the brain's pleasure system: the mesolimbic dopamine system (food, sex and other things trigger this system, making us feel good). When addicts repeatedly overuse drugs, it overstimulates their pleasure system and they lose the ability to control and satisfy their cravings.