The short answer
A negative thought on repeat is usually two separate, well-documented mechanisms compounding each other. The first is negativity bias: the brain is built to weight negative information more heavily than positive information of equal size, a principle established across decades of research. The second is the Zeigarnik effect: unfinished mental tasks intrude on attention far more persistently than tasks that feel resolved. Put together, negative and unresolved is close to the worst combination for a thought that wants to stop repeating.
The research
Unfinished mental tasks intrude on attention far more persistently than tasks that feel resolved — the mind keeps an active flag on anything it registers as unresolved until the task is completed or abandoned.
— Bluma Zeigarnik, 1927
Why negative content gets more weight in the first place
Baumeister's review found the negativity bias showing up consistently across an unusually wide range of domains — bad first impressions form faster and resist correction more than good ones; bad relationship events carry more weight than good ones of equal size; negative feedback gets processed more thoroughly than praise. The likely evolutionary logic is straightforward: missing a real threat was historically far more costly than missing an opportunity, so a brain tuned to overweight potential danger, even at the cost of some false alarms, was the safer bet across a long evolutionary timescale.
Why unfinished thoughts specifically won't let go
Zeigarnik's original research found that people remembered interrupted, incomplete tasks noticeably better than tasks they'd been allowed to finish — the mind appears to keep an active flag on anything it registers as unresolved, and that flag keeps drawing attention back until the task is either completed or the mind is convinced it's been abandoned. A worry about an email you haven't sent, a conversation that ended awkwardly, a decision you haven't made yet — all read as "unfinished" in exactly the way that keeps intruding.
Why this combination is so sticky
A negative, unresolved thought gets pulled toward the front of attention by both mechanisms at once: it's weighted more heavily because it's negative, and it keeps resurfacing because it's unfinished. Neither mechanism, on its own, requires anything to be wrong with you — they're standard features of ordinary cognition, not signs of a disorder. What varies between people isn't whether these mechanisms exist, but how often, and how intensely, the two combine.
Want to see this pattern in your own thinking, not just in the research?
What actually reduces the repetition
Since the Zeigarnik effect is specifically about unfinished tasks, one workable lever is giving the mind somewhere to register the thought as "noted" without necessarily resolving its content — the difference between a thought that's been marked and set down versus one that's still fully open. A tap, a written note, a voice memo: something external that stands in for closure, even when the actual situation isn't closed yet, tends to quiet the repeat-intrusion mechanism more than trying to think the thought all the way to a real resolution in one sitting.