The action is designed to be easy and fast - a 5-minute lesson, a single swipe, a quick post. Low friction lowers the activation energy needed to begin. The simpler the action, the more often it happens.
Design: Low-friction micro-task Neuroscience: Dopamine anticipation

Unpredictable rewards produce stronger compulsive behaviour than fixed ones — the same mechanism that makes gambling addictive. Apps deliberately randomise reward size so you keep returning to find out.
Design: Variable ratio reward schedule Neuroscience: Dopamine spike on uncertainty

Streaks reframe engagement as accumulated value - value you will lose if you stop. Loss aversion (the pain of losing is twice as powerful as the pleasure of gaining) makes breaking a streak feel significantly worse than the benefit of a single day felt good.
Design: Progress artifact + loss framing Neuroscience: Loss aversion (Kahneman)

Seeing a friend ahead does not make you stop — it makes you work harder to catch up. Both users increase engagement simultaneously. The app benefits from the competition it created.
Design: Visible leaderboard / friend feed Neuroscience: Social comparison + status anxiety

Notification timing and wording are A/B tested. "Your streak is at risk" was selected because it produces the highest tap rate — combining loss aversion with guilt in a single sentence.
Design: Timed push + loss framing Neuroscience: Anxiety + obligation response

The user did not choose to open the app — the notification and the streak made it feel necessary. The cycle repeats daily not because the app is valuable, but because the cost of stopping feels too high.
Design: Notification-to-session pipeline Neuroscience: Compulsive checking behavior