B.F. Skinner discovered that animals respond most compulsively to variable ratio reinforcement schedules -- rewards that come at unpredictable intervals after an unpredictable number of actions. Games have been using this principle since the earliest RPGs. Kill enemies, sometimes they drop loot. Open chests, occasionally one has something amazing. The unpredictability is the hook; if you knew exactly when the next reward was coming, the compulsion would collapse. The best Skinner box designs disguise the box with enough genuine gameplay that the reward loop feels earned rather than manipulative.