Eating something that you enjoy or being stimulated sexually can cause dopamine levels to increase. In these graphs, dopamine is being measured inside the brains of animals. Its increase is shown in response to food or sex cues. This basic mechanism of controlled dopamine release and reuptake has been carefully shaped and calibrated by evolution to reward normal activities critical for our survival.
As a result of scientific research, we know that addiction is a disease that affects both brain and behavior.