A reward is a great way to encourage someone to do something. For example, if you offer a $300 reward to find your lost dog, people may be more likely to look for and return your dog. Or, if your parents offer to reward you for keeping your room clean or getting good grades, you have an incentive to do it.
Wired to want more...
Our brain has its own reward system. When we do certain things, the brain "rewards" us by making us feel good. The brain reward system is a brain circuit that causes feelings of pleasure when it's “turned on” by something we enjoy (see figure), like eating good food or being in love.
Whenever this reward circuit is activated, our brains note that something important is happening that's worth remembering and repeating. Drugs activate the brain reward system in a similar manner. However, most drugs set off a surge of the brain chemical dopamine, so they produce a much stronger and longer-lasting “artificial” pleasure sensation than natural highs.
...or just to use more?
The effect of such a powerful reward strongly motivates people to take drugs again and again, even when they no longer really want to. That can happen because drugs can actually reprogram the brain, so that every time a person takes the drug, the effect is a little weaker. So they have to take more and more of it to get the same feeling.
Eventually, a person can become addicted to the drug and use it compulsively, not so much to feel good but to keep from feeling bad. That’s the “sneaky” part of addiction.
NIDA provides lots of information about the how drug abuse targets the brain’s pleasure center: