This is how we're wired as a species, and frankly more than just a species. We don't just look at what we have and what we want, we look at what other people have and ask ourselves why they got it and we didn't. At a basic level it's an instinct that exists in lower primates, and in us as well.
The difference with us is that we can learn and understand the reasons why one person gets a "grape" and the other a "cucumber"... and those answers are often not just unsatisfying, but infuriating.
It exists even in dogs. It's well studied and not new even.
And it's entirely rational, not merely universal or understandable.
Chris Rock has a joke about the same sort of argument that some white people try to make about black people having it objectively and absolutely so much better now. True. They do. So would you trade places? Even a Walmart greeter wouldn't trade places with me in a million years "and I'm rich!"
If "objectively better off than yesterday" is all that anyone "rational" should care about, then I'll take over your valueless extra 400k (you can keep 100). After all, your dad had to get black lung in the coal mine and die at 47, all you gotta do is play around on a computer in an air conditioned clean office. You have it objectively 1000x better. You (the parent comment not you) should be fine with this. After all, it does not matter at all what I do with that 400k, or what I did or didn't do to get it, or what you did or didn't to to only get 100.
https://www.youtube.com/watch?v=meiU6TxysCg
The difference with us is that we can learn and understand the reasons why one person gets a "grape" and the other a "cucumber"... and those answers are often not just unsatisfying, but infuriating.