I saw your title and got excited and typed what I did below answering that specific title. It might not be completely relevant but I think it's fascinating so I'm posting it anyway.
I listened to this episode of the Game Design Roundtable with guest Nicole Lazarro, a psychologist that studies the "why" of fun as it relates to games.
Through researching emotional reactions while people played games, Nicole identified game components that elicited strong emotional response from players. She sorted these components, and came up with this slightly confusing graph called the 4 Keys 2 Fun.
To summarize, she breaks down the concept of fun into these 4 components, and I'll use the game Rocket League to contextualize it:
Easy Fun: The "bubblewrap" of games. This is stuff like open world exploration, roleplaying opportunities, enjoying how the controls work, and so on. In Rocket League, easy fun is playing around with the simple controls and making engine noises as you drive. It's also when your opponent's score by nudging the ball into the goal and all the cars blowing away.
Serious Fun: Play as therapy. This is about repetition, about one more match and that's it. This is about worrying about your win loss ratio, or looking up how other people do it online. Rocket League serious fun is about experimenting with the way you play, playing match after match as your grind mastery.
People Fun: The kind of fun you get from playing with other people. It's rubbing your friends nose in it when they lose or the entire couch erupting in laughter during a weird play. People fun is Rocket League's multiplayer.
Hard Fun: Hard fun is about playing to win. This is when you get really serious about the game and develop mastery over it. This is the double-fist in the air moment of victory when you score a goal.
Each key corresponds to a different emotional range, and a really successful game is going to pull these strings at different times to pace the player and keep them engaged. The best games will do this by virtue of their design, and not as the psychological skinner boxes that are mobile games.
