Drugs that cause addiction change the ability of receptors in your brain to cause you to feel pleasure. There are a few ways this can happen. Depressants often work by increasing the affinity of a receptor for a small molecule called GABA (gamma-aminobutryic acid). Stimulants make you feel happy or better than you would otherwise. They can do this in a few different ways, but two common ones are to either cause more dopamine to be released or to prevent the reabsorption of dopamine so that it stays around and keeps you happy for longer. Narcotics act in a similar manner to stimulants in that they mimic the molecules that make you happy normally.