I tend to look at it like this: humans have a certain base capacity for worrying - for noticing and suffering problems - that's independent of their situations and the nature of problems themselves. Solving problems and improving our lives doesn't reduce worrying, as much as changes the nature of it.

The poor person sees the rich person complaining, and thinks to themselves those are all "first world problems", really nothing to be worked up about. But should said poor person work their way up and become wealthy, they may discover they're suddenly preoccupied by those same problems they used to dismiss as irrelevant.

There's also something to be said for being in vs. out of a certain situations. As a young adult, there ware many weird things I saw older adults worry about - or perhaps make choices based on (what I thought were) weird ethical priorities. Couple years later, with a spouse and two small kids, I find myself worrying about those same weird things, and making similar ethical choices, which suddenly feel entirely justified once you're responsible for and worrying about the physical and mental well-being of a small human under your care.

But yes, on top of that, there's also "having an eye for detail", varying degree of tolerance for minor annoyances, etc. - whole range, up to and including OCD.