" humans find the sweet spot sandwiched between “too easy” and “too difficult” irresistible. It’s the land of just-challenging-enough computer games, financial targets, work ambitions, social media objectives, and fitness goals. Addictive experiences live in this sweet spot, where stopping rules crumble before obsessive goal-setting. Tech mavens, game developers, and product designers tweak their wares to ensure their complexity escalates as users gain insight and competence. "

― Adam Alter , Irresistible: The Rise of Addictive Technology and the Business of Keeping Us Hooked