The best game creates a balance between difficulty and reward. While it challenges you, it needs to give you positive reinforcement.
Incentives to stay in a game include:
-collectibles (Skyrim- because I love my glass/malachite swords and armor),
-exploration (Skyrim) (The OP's Universalism),
-story (Mass Effect),
-physical stimuli (breast fondling Oculus extention),
-comedy (Borderland's Claptrap, Goat Simulator),
-encapsulation (Flappy Bird) (Halo),
-determination (The Impossible Game) (Competitive),
-socialization (Destiny *trollface*),
-and education/training (EK would say AC).
What makes a game great is as complicated as the mind itself, but since the games must pander to the average consumer, it's easier to do, though not so much. A game that can enamor the player and subject them to these feelings is the first step. Some aspects of a game would be counteractive (education and mind-numbingness) and/or can't be experienced at the same time -- and then people can get emotionally or intellectually drained -- so to prevent this, the game should be more encapsulating than anything.
Moments of hilarity, triumph, or epicness should never fatigue the player, or that'll hamper its playability -- perhaps if it were an app, but how would an app achieve that?... or better yet, what app has?
Socialization would be defined as the ability to create positive and abundant social interaction.
Immersion has essentially been the topic of this post, although, how much negative feedback there is is also part of the equation.
Edit:
Pride is a major factor as well. Having a distinguished skill-tree/skill-set and collectibles is a facet of pride, along with K/D and creation. Modes such as Forge or Far Cry's Map Creator and game such as Sims or Minecraft, get you attached to your creations - pride.
Sometimes, nothing is better than the satisfaction of completing something, patting yourself on the back, and then shaking it off for the next set of challenges.