Slot machines run on the most addictive reward pattern psychology found

B.F. Skinner found that rewarding a behavior after an unpredictable number of tries produces the highest, steadiest response rate of any reinforcement schedule — and the one hardest to extinguish. There's never a safe moment to stop, because the very next pull might pay off. Slot machines run on exactly this variable-ratio schedule, by design.

— Charles B. Ferster and B.F. Skinner, Schedules of Reinforcement — Appleton-Century-Crofts, 1957