system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What goes on in the brain during an intrusive thought?
gpt
Your brain doesn't stop. period. It constantly works to keep you alive.

But how it works is by exciting neuron groups. So when you are "remembering" something or "thinking" about a problem, the related group of neurons "lights up", you can actually see this on a MRI scan.

Unfortunately, unlike machines, sometimes you get a faulty activation, which lights up a unwanted patch, which becomes the unwanted thought.

There is also the fear explanation, you see a cliff, and you keep having thought about jumping/fall over it. some believe this is the self preservation mechanism kicking in, warning you to back off dangerous places.