Addiction changes brain circuitry, making it hard to "apply the brakes" to detrimental behaviors.
In the non-addicted brain, control mechanisms constantly assess the value of stimuli and the appropriateness of the planned response. Inhibitory control is then applied as needed. In the addicted brain, this control circuit becomes impaired because of drug use and loses much of its inhibitory power over the circuits that drive responses to stimuli deemed salient.
As a result of scientific research, we know that addiction is a disease that affects both brain and behavior.