I know it's a tired trope to throw around at this point, but it just shows there are countless variables that factor into one's mood and disposition and sense of motivation even before you dig down to the neurological and biological levels. Leave a healthy, well-fed, "rich" rat or chimpanzee alone in a low-stimuli, not very large room for long enough and they'll often become depressed. Embed someone born in a wealthy country into an indigenous group and they often report feeling more alive and not depressed. Do the reverse and you often get the reverse.

And of course many people feel all these things not due to any sort of stagnation they have any personal control over but due to neurological systems which may be difficult to alter. Or both.

I think the original reply post may have partly misinterpreted what the commenter was trying to say. The hypothesis being discussed is something like that even if you reintroduce the rat back to its previous happy environment, their potential predictive processing dysfunction may cause them to not be able to do anything but treat all stimuli the same: not worth caring about or trying to derive enjoyment from.

What country is this? Your domain seems to indicate Kazakhstan, but they have one of the highest suicide rates in the world, so I suppose that's not it?