When a surprising reward occurs (or, as they say in the literature, a "salient" one), a flood of dopamine is released. That flood is like adding fuel to a fire, and the brain activity is intensified along the dopaminergic pathways.
While that's happening, new memories are being formed, too, encoding the current input from the senses as a stimulus. It's the pattern of whatever the brain was sensing and thinking while that new reward was experienced.
From now on the link is made, and whenever this pattern (this stimulus) is present, the memory of the reward is activated. This happens even before the reward itself is consumed. It's what behaviorists call conditioning.
I'll use ice cream as an example of this process. It's full of creamy sweetness (which qualifies as an excellent reward), but pretend you don't know that. Imagine you've never before in your life so much as heard of ice cream. Then take a lick.
The sugar should trigger an automatic reward signal in your brain, and your dopamine system will detect it and light you up right afterwards (Trial 1, in the figure below).
The experience of the sweetness is intensified by the heightened activity, so the memory will be vivid. The sight and smell of ice cream alone will be enough to remind you of its taste, and how good it made you feel.
The reward of ice cream is now conditioned. This is the experience of "pleasure," but I wouldn't call it fun yet. There's one last step in the process.
(Fig. 2) The dopamine release moving back in time over four trials until it reaches the furthest reliable signal of reward.
Because you've been conditioned, seeing or smelling ice cream (the stimulus) again will now activate a dopamine response even before you taste it (Trial 2). Not in the original area (those neurons closest to experiencing the taste have already begun to expect it, and aren't as surprised). Instead, new areas that weren't involved in the exact moment of the ice cream reward are being lit up.
Remember, this is now happening before you've actually gotten the ice cream. The dopamine driven brain fire is happening while you're standing IN LINE for the ice cream. The memory and stimulus is being recorded progressively further out across time (Trial 3). It's your brain encoding a prediction of the reward. Even though standing in line is not at all the same as eating ice cream, you're doing it because you're compelled by the memory. That's what we call "fun."
"But wait," you say, "standing in line sucks!" (And you're right.) It sucks for you and me because we've done it too many times, but try to imagine the first time you stood in line for ice cream. I'd bet you were giddy with expectation and could barely contain your excitement. That's the dopamine system at work. It's intensifying the experience by making the memory of the ice cream so vivid that you can almost taste it, while laying down a memory of walking up to the ice cream shop, looking over the flavor choices, and trying to decide which topping (chocolate fudge or whipped cream?)
All of these experiences and choices are new processes in your brain, and all of them are receiving an novel link to the reward of ice cream. That's keeping the dopamine flowing, and making it all feel good. As long as a reward can be predicted by a new stimulus, the dopamine system will keep causing the excitation needed to record it, and the whole experience carries a compelling feeling and expectation of the reward. It is not the reward itself, but the fresh process of learning to get it and the motivational feeling of wanting the reward.
So that's what I've come to think of: the dopamine-enhanced experience of reward while a new predictive stimulus is being mapped out. A human software interface. Although every brain is born different, some nearly immune and others very susceptible, the neurons and dopamine are always there. With a little deduction it's a tool of strong game design. Keep the dopamine flowing by making sure you provide salient rewards and just enough operant complexity to keep it all from ever being mapped out, but not too much or else you shut them down.
It explains why you're willing to keep clicking "reconnect" when the WoW server goes down, and it explains why you can rehash old mechanics so long as you're selling them to an audience who hasn't already learned them (i.e. children, new gamers.) It also establishes how splitting your rewards across many brain systems (social, aural, mechanical, visual) is such a compelling combination. It's a bit surprising that Vegas hasn't figured out how to turn gambling into more of a social game for people besides the high rollers.
(Fig.3) A momentary depression occurs when an expected reward is absent.
Once everything that can be reliably linked to ice cream has been wired up, the dopamine system goes quiet. The neurons involved have become acclimated; they no longer get excited when they sense the stimulus (Trial 4). They simply expect their reward of the ice cream in due time. With nothing new to learn, they no longer trigger excitation. Standing in line for ice cream now sucks.
Even though ice cream itself is still delicious, the line is simply work you do to get it. Things "just don't feel the same anymore." Even more than that, those dopamine neurons are still watching, and if the predicted reward doesn't show up when they expect it, the whole system flat-lines. When they fall on their face, you have no motivation to continue, and quite possibly feel pissed. It's a theory of boredom, as well as fun.
It hints at addiction too, but in doing so it resolves the difference between an "addictive" game and a drug. It has to do with the strength and means of the dopamine release. Almost all addictive drugs like methamphetamine affect dopamine, and can trigger its immediate release at tenfold normal levels. Given how this reward system operates, you can imagine how strong this conditioning is. Games only trigger the release of dopamine through normal sensory channels, and at a more healthy and sustainable rate.
By this theory, the largest dopamine rush accompanies the first few times you play. If you've ever felt a compulsion to rush home and play a game all day, you are likely getting a hint of what a strong addiction feels like. Afterwards, the combined effects of acclimation and inhibition curb further dopamine release.
Can a game lay down enough conditioning over time that begin to match the levels of reinforcement seen in drugs that immediately release dopamine? Perhaps, and we have strong profit motivation to see if this is true. It's reasonable to assume some people can be trained to compulsively play games.