Why are slot machines so addictive?

Slot machine Silly: This is an interesting question and relies upon a basic principal in psychology known as variable reinforcement. When you unpredictably reinforce a behavior (playing the slots) with a positive outcome (winning money) you cause that behavior to be repeated over and over in the "hope" of getting the reward. Rats can be trained to push a bar non-stop if they're fed periodically as a reward.

Compulsive gambling: Slot machines are one form of gambling that may lead to compulsive gambling. Mutual support helps; consider attending a meeting of gamblers anonymous.