There is a difference between fear and obsession.

In example, I spend probably 0% of the average day thinking about trapdoor spiders, chainsaw murderers, or the concept of a wood splinter being lodged in that space between your fingernails. That number goes up if I am directly confronted with the object in question or a situation alluding to it, of course, but on an average day, I don't think about them.

Those are quite nasty stuff, there is merit in bringing them up, but in any given conversation, I'm not going to. Because they aren't on my mind. They don't occupy my brainspace. It is an interesting thought exercise to think of things I don't think about.

Someone who often brings up a concept without prompt... It is natural to assume it is occupying a significant amount of their brainspace. Their free time is spent thinking about it. Now, not everyone who spends every waking hour thinking about something is actually obsessive about it in a societally negative way, but a much larger fraction of them will be, than the average person to whom the concept does not occur at all.

The person on the street corner screaming at passers-bys "don't give into the urge to kill people who wear green ties! Resist the urge!" is much more likely to wind up being a murderer of someone who wears green ties, than the guy walking by him saying "fuck off, nutter".