Rats in a casino
From Adam Alter’s Irresistible: Why We Can’t Stop Checking, Scrolling, Clicking and Watching:
Juice refers to the layer of surface feedback that sits above the game’s rules. It isn’t essential to the game, but it’s essential to the game’s success. Without juice, the same game loses its charm. Think of candies replaced by gray bricks and none of the reinforcing sights and sounds that make the game fun. ...
Juice is effective in part because it triggers very primitive parts of the brain.