animal brains are designed to create flashes of pleasure when the animal does something important for its survival, and small pulses of the neurotransmitter dopamine in the ventral striatum (and a few other places) are where these good feelings are manufactured. Heroin and cocaine are addictive because they artificially trigger this dopamine response. Rats who can press a button to deliver electrical stimulation to their reward centers will continue pressing until they collapse from starvation.
