Why do we get mentally stuck on unfinished tasks?

When a task is incomplete, the brain may keep returning to it because the goal has not been closed off. This can make unfinished work feel more mentally intrusive than completed work, even if the unfinished task was simple. The effect was described by psychologist Bluma Zeigarnik in the 1920s after observing that waiters remembered unpaid orders better than paid ones. Although the details can vary, the broad idea is that open goals can continue to occupy attention until they are resolved.