Let's talk about single-player games.
Take it from a novice game designer: the player element can be terrifying at times. We pretend at first that games are an independent construction. But soon reality comes crashing down; the player, another human being we might not even know, is an integral part of the experience we are trying to construct.
And try as we might to borrow understanding from film, theater, and prose, those principles of passive media all but cease to apply when the player element is taken into account. You must leave part of the enjoyment in the hands of someone else! The player isn't just along for a ride, the player is a power in and of themselves! It's chaos!
So how do you deal with this player element, this entity that claims to be stronger than your level design, your enemy AI, and even your bosses? What can possibly stand toe-to-toe with the enigmatic player?
Treat the game itself as an entity on par with the player, and things start to make a lot of sense. The player is trying to "beat" the game. The player is taking action against the game itself on a subconscious level. The player is taking the whole game seriously. So it stands to reason that the game, as an entity, should take the player just as seriously.
This symbiotic pseudo-relationship informs a number of structural ideas for video game design.
How does the game fight back against the player's advances? By forcing the player to change tactics. By making a significant change in the current gameplay that forces the player to stop, think, and take more care in what they do. The game, in the player's eyes, is in charge of these things. Just as the player messes with the game, the game should mess with the player with gradually increasing strength.
In a story sense, the game can be considered the antagonist to the player-protagonist. And just as the player empathizes with the player character, the game-antagonist can perhaps personify itself in the main villain. And as the protagonist and its antagonists exchange blows over the course of a story, so too do the player and the game participate in a series of actions and reactions.
If the game-antagonist's desire is to stop the player-protagonist, then the game should exhaust every weapon and obstacle available in the game's world. Each level of gameplay should exhaust every obstacle reasonably available in that physical space, and the course of the game should use up everything in the entire game world. The player must triumph over all of these and reach the end of the line, a place where the game has nothing left and is officially "beaten."
As game designers, we want to make the player ultimately satisfied at the game's end. But in order to do that, we need to raise the stakes by fighting back a little, by making the player stop, think, and (most importantly) care. If we treat the game as an entity on par with the player, the means to challenge them start to become clear.