Dopamine does a lot of things, but you're probably most familiar with it as the chemical your brain uses as a sort-of system of in-game gold coins. You earn the reward for certain behaviors, usually "lizard-brain" type stuff—eating a bowl of pudding, for instance, or finally making out with that cute person you've had your eye on. And, as you've probably heard, there's some evidence that we can get addicted to that burst of dopamine, and that's how a nice dessert or an enjoyable crush turns into something like compulsive eating or sex addiction.
Neurologist Robert Sapolsky puts an interesting twist on this old story, though. What if it isn't the burst of dopamine that we get addicted to, but the anticipation of a burst of dopamine? It's a small distinction. But it matters, he says, if our reward system is based less on happiness than on the pursuit of happiness.
For more on this, check out David Bradley's post on this video, which also links back to a more-detailed discussion of the basics of dopamine addiction.