I think in its simplest form, games are just purely time-killing entertainment. Back when Pong was new, you could kill time playing against the game itself or a friend. If you wanted to try to get anything more out of it, you could try to set up some goals -- I'll try to beat my friend this many times in a row, I'll try to get a high score, whatever. You'll see this a lot on Flash games on sites like Newgrounds or Kongregate. Simple games that're fun and kill time.
When games became something more, namely platformers of one variety or another, the reward system was built in. Now you're objectives were more definite and built into the game play itself. Donkey Kong gave you a definite, greater goal than just who could beat who the most. You had to beat that damn great ape, and you had to surpass these obstacles to do that.
It was the same for a lot of games thereafter, only the goal evolved again. This time you still had a boss to beat, but you had larger levels and more and more difficult obstacles to overcome. Mario left Donkey Kong alone to beat up some turtles and mushrooms and Bowser while making his way from one end of the level to another. Same with Contra, Metroid, Mega Man, you name it.
In those kinds of games, you completed your goal, your brain sent happy waves of happiness and you felt like you accomplished something, especially after a particularly difficult level.
Not only that, but there was all kinds of crazy imagination that went into the concept of these games, the sprites you controlled, the baddies, the worlds themselves, and that's just all kinds of cool no matter what medium you're looking at.