Substance abuse
One idea about why people use drugs is because of their genes. When a person's parents or grandparents have been addicted to drugs, alcohol, cigarettes, or even coffee, the same genes that made them become addicted could be passed down to any of their children. When someone is born with genes that increase addiction, it causes their brain and body to become dependent on drugs very quickly. But even if a person's genes make it easier for them to become addicted to drugs, that person still needs to try a drug first, before they can become addicted to it. This idea also means that different people can have a harder or easier time stopping drugs after they are already addicted.