Why does a habit keep running after you stop wanting what it brings?

Because a habit has stopped asking whether you still want the result. Early on, behavior is controlled by the expectation of an outcome: you do it to get something. After enough repetition, it can be elicited by the situation itself, autonomous of the outcome.

Neuroscientists separate the two with a cruel little test called outcome devaluation. Train a rat to press a lever for sucrose, then make the sucrose nauseating with a taste aversion. A rat still working for the sucrose should stop pressing. A rat on autopilot keeps pressing. In one classic experiment, normal rats trained this way kept pressing, while rats with damage to a region called the dorsolateral striatum refrained once the outcome was devalued. The authors concluded that this region is necessary for habit formation.

In everyday life, the clash shows up as a capture error: the habit prevails over what you consciously meant to do. And the habit may not be gone when it seems to be: the basal ganglia appears to remember the context that triggers it and can revive it when the triggers return.

Here is the catch. The tidy rat story has been hard to reproduce in people. A team reported five failed attempts to make healthy humans form inflexible habits just by overtraining them. So the same word, habit, covers a clean result in rats and a much murkier one in us.