One way of exploring our need to play is to dig deep into the biology of the gaming experience. There are a number of key chemicals that can be identified, such as the neurotransmitter epinephrine (or adrenaline), which is the underpinning of excitement -- the most primal of the emotions of play.
Games of chance and competition add to this raw excitement a winning state that produces feelings of elation that cause the victor to punch the air or raise their arms, what Suits describes as "the truly magnificent exhilaration that can be produced only by a supreme triumph". There is no word in English for this emotion, but the researcher Paul Ekman (2003) notes that in Italian it is called fiero, the personal triumph over adversity. Since this word is unfamiliar to most English speakers I will use the term triumph as a synonym.
The experience of triumph is intimately involved with some of the most popular forms of play and this emotion, and its watered-down version satisfaction, can be correlated with the neurotransmitter dopamine. This reward chemical is principally released by the reward center of the brain's limbic system, the nucleus accumbens, and is involved in emotional experiences of satisfaction and triumph.
This neural apparatus is vital to the formation of behavior, and can also generate compulsion via reward schedules (or schedules of reinforcement) of the kind identified by B.F. Skinner (1938) and Charles Ferster (1957).
Game mechanics based upon these systems are endemic to digital games, as John Hopson (2004) and others have observed, especially in the case of computer role-playing games and MMOs. While the player maintains interest in what the game is asking them to do, they will merrily jump through whatever hoops they are pointed towards, provided there is some reward to be paid out.
However, when they begin to lose interest they will become aware that they are being asked to perform a series of highly repetitive tasks in order to achieve some measure of progress. Players call this the grind and the associated activity grinding, and compare it to a metaphorical treadmill. The comparison is apposite -- but it is important to remember that the hamster often enjoys running on their wheel.
When a player is grinding, they are expected to keep accumulating an in-game resource (usually either money or experience points) in order to reach progressively higher targets, each target affording the player a reward in terms of increased power or capability. Economist Edward Castronova has gainfully compared this to Camus' account of the myth of Sisyphus, the mythological Greek king who was condemned to roll a boulder up a hill repeatedly, only to have it roll back down again.
But Castronova notes that it is not quite this futile in a game with grinding: when the grinding player reaches the top of the mountain, the stone goes over the top and rolls into the next valley, and as Sisyphus goes to roll it up the next mountain he discovers he has become more powerful. Again and again, the grinding Sisyphus conquers one mountain to find another behind it -- but all the while, they have the sense of achievement from having gained a little bit of power in the process.
Grind is often singled out for criticism, but this is because when players notice they are grinding, they have already lost interest in the activity they are being asked to perform. Pragmatically, those games which rely on reward schedules to structure their play can maintain interest for radically greater lengths of time than those that do not -- and provided what the player is asked to do does retain some interest, the grind is precisely what maintains the player's interest. While some game designers try to develop mechanics which avoid the grind, games that make use of grinding are becoming increasingly significant in the commercial market for digital games.
There are sound reasons for certain players to prefer games which include grinding, since the mechanics behind grinding usually afford progressive advantages to players for continuing to play, and this means that players with a lack of skill in a particular relevant area can compensate by simply grinding to increase power ("level up").
The sense of triumph may ultimately be less because of this self-adjusting element of difficulty, but this also means that success is not restricted to the players with the necessary skills (or tenacity) to overcome. Furthermore, players open to grinding can still achieve triumph by aiming for thoroughness -- completing collections, for instance, or doing everything possible that the game presents as a goal.
Grinding is an important part of the appeal of certain digital games, namely those which utilize reward schedules to structure the play. Although these mechanics began in fantasy role-playing games, they are now found in a great many other games. The appeal of the car simulator series Gran Turismo (Polyphony Digital, 1997 onwards) lies in part in the capacity to grind races in order to earn money and thus buy bigger cars.
The Modern Warfare franchise (Infinity Ward, 2007 onwards) has applied reward schedules to the already successful first person shooter (FPS) genre of digital games and as a consequence set new benchmarks for sales, selling 20 million units in a market that previously topped out at about 8 million. A great many games now directly target their players' reward centers via reward structures and grind.
The reward center of the limbic system also has incredibly close structural ties to another part of the brain, the orbito-frontal cortex which lies in the brain at a position just above the eyes.
This is essentially the rational decision center of the brain, and there is no other part of our neural architecture more closely tied to the nucleus accumbens, where most dopamine is produced. Making a good decision is pleasurable, and indeed this explains why solving puzzles is enjoyable: when we find the solution, it triggers a release of dopamine.