An unfinished task nags at you far more than a finished one

In the 1920s Bluma Zeigarnik noticed Berlin waiters could recall unpaid orders in vivid detail but forgot them the instant the bill was settled. Follow-up experiments found people were much more likely to spontaneously recall an interrupted task than a completed one. The effect is now used deliberately in cliffhangers, to-do lists, and study techniques that break work into unfinished chunks.

— Bluma Zeigarnik, Zeigarnik effect — Psychologische Forschung, 1927