Why do you suppose many adults pine for the “good old days,” either an earlier era in history or their own youthful high school/college days? Is this generally a harmless or harmful thing? Why?

Maybe that was the peak of their existence. The time at which they'd accomplished most things, when they'd met the greatest people, when they'd fallen in love and fallen out.

It is harmless as long as they do not live in the past. Reminiscence is not a bad thing. But they have to remember to live in the present.

However, it's not the same for all adults.

Not everyone has had an eventful youth. For some, 'the time of their life' arrive at a later point. For others, it doesn't happen at all.

now, of course, the quality and the seasonability of these winter dreams varied, but the stuff of them remained.   they persuaded dexter several years later to pass up a business course at the state university—his father, prospering now, would have paid his way—for the precarious advantage of attending an older and more famous university in the east, where he was bothered by his scanty funds. but do not get the impression, because his winter dreams happened to be concerned at first with musings on the rich, that there was anything merely snobbish in the boy. he wanted not association with glittering things and glittering people—he wanted the glittering things themselves. often he reached out for the best without knowing why he wanted it—and sometimes he ran up against the mysterious denials and prohibitions in which life indulges.

there you go thats what we are missing