As a hobbyist game designer, I quickly learned a simple truth about game players:
Players always know whether they are having fun but rarely know why.
If you don't believe this, read the fan forums for literally any game you can think of. You'll see all kinds of suggestions, 80% of which would clearly make the game less fun. But when someone says, "I don't like it when this happens," then they're stating a personal opinion. It's not a debatable point-- they are certainly not having fun. This is a bit ironic. How can they know what they like but not why they like it?
The answer is that while humans are capable of self reflection, they aren't actually that good at it, nor do they usually enjoy it. Humans excel at pattern matching and general recognition problems. Not coincidentally, the vast majority of fun in games comes from what could be abstractly labeled "pattern matching".
Pattern matching is more than just seeing the Virgin Mary in a grilled cheese sandwich. In an abstract strategy game like Chess, it involves recognizing a board position as similar to one you've played before, so you have an idea for how to approach it. In Poker, you learn to recognize how a player's bids correspond with their potential hands. Basketball and football players need to recognize openings in the opposing team's defense. Party games like Charades and Apples to Apples are nothing but pattern matching: can you connect the dots between someone's gestures and a previously known phrase?
So I was intrigued after reading an article by Mark Rosewater on resonance in Magic: The Gathering. A game mechanic has "resonance" if it reminds the players of already known concepts. If you see that a dragon has the special power "flying" or giant has "trample", you don't even need to know how the game works to appreciate the design. We know that dragons fly, giants are large, zombies are slow, merfolk swim, and so on. It's just fun when a game does a good job of embodying its theme! And game mechanics with resonance are also easier to learn, since you can apply your standard intuition to situations they apply in. You would assume that a creature with flying is harder to attack and has an easier time attacking other people. A trampling giant probably causes more damage on average than another creature would. Zombies are probably weak overall, but easier to use as minions than something like a Dragon.
I confess this lesson took me a long time to learn as a game designer. The first games presented players with interesting decisions but totally lacked theme and resonance. This made them as entertaining as sudoku, which puts them the same realm as, "things I'm willing to do while in a train." Not exactly a rousing endorsement. But just because of how our brains are wired, humans just like seeing connections between abstract actions and known concepts. When kids play cops and robbers, "house", or pretend a doll is a real baby, they are taking enjoyment from resonance. Since even the very young participate in imaginary play, it suggests this enjoyment is somehow wired into our brains. Some people think of games as escape from real life, but it's specifically when games are connected to life that they are most enjoyed. A game doesn't need dragons, ninjas, aliens, or Roman emperors to be fun, but these things help a lot. Especially the ninjas.