One example would be to grow up into a world you no longer understand.
Some would bother to mourn the "new" world they're in, when it's more accurate to say that their past was naivete yielded from abandonment: to be a deaf dumb mule, to not see reality, to not be taught.
Instead of confronting one was uneducated, unprepared, ignorant, instead most aspire to crawl back in.
But after you live long enough, you can't ignore anymore. No more placating, no more retrospectively harmful safe-spaces.
Still, it is common practice to idealize the "simple" past than to instead realize your time was wasted, you were too stupid to see how misled you were, and you now must catch up unless you would rather suffer needlessly.
So I find nostalgia distasteful. Because instead of crawling backward, one can realize that everything is now; expand upon the realities you want to be in!