I was talking to a clinical psychologist once, and they were talking about how certain kinds of intrusive thoughts can be self-reinforcing. Note that I haven’t done any independent reading of this idea, so I don’t know it it’s true, but it’s not implausible from what I remember of neuropsych.
Basically, the idea was you have a thought which includes content you find upsetting. Your brain sees “oh, this got a reaction! It must be important to pay attention to!”, so those neurological circuits get reinforced, which means they trigger slightly easier, which means that kind of thought is likely to recur, which reinforces the circuit, etc etc until it hits some kind of equilibrium dictated by your environment, genetic factors and suchforth.
This idea is particularly important in therapies such as acceptance-commitment therapy, which I don’t know a lot about (not being a clinician), but apparently is useful for some issues.