Studies suggest that people with depression may have lower baseline dopamine levels and are less sensitive to rewards, which can make them more vulnerable to addiction as they seek out rewards to feel normal. Chronic drug use can result in a dopamine-deficit state, causing people to feel restless and anxious and increasing their motivation to use drugs again.
The brain produces dopamine in response to pleasurable activities, but when we bombard our brains with too much dopamine, it leads to down regulation, and chronic dopamine deficiency. This can cause a persistent focus on the drug of choice in order to recreate the feeling and return to normal homeostasis.
People with depression may be more vulnerable to addiction, as they seek out rewards to feel normal, due to lower dopamine levels; and chronic pain sufferers need to go beyond their baseline pain before experiencing relief. Pleasure and pain are processed in the brain, where any reward-seeking behavior has a cost, which is the come down and the subsequent pain.