As others have said, cows long for high school because it's the last time in their life they had a good time, and it's not just high school, it's the life they led back then, when all they had to do to succeed or be judged "successful" by peers and parents was to wake up in the morning, go to school, watch cartoons and not set the drapes on fire. That was it. Do that and you were patted on the head, told what a good kid you were and life was pretty cush.
To them, this was the BEST part of their life because once those days were over, they were left stranded.
They failed to adapt to "adult" expectations where simply going through the day not getting in trouble wasn't enough to be considered a success, that's where a lot of these folks have stopped maturing, they're emotionally 15 or 16 years old, or worse, and can't handle the stress of life, so they withdraw into their little fantasy worlds where they're still free to be their old selves, just like in high school, free of obligation, free of expectation, free of all that stress and worry.
That's why they do it, they don't really LIKE everything that came with the high school years, they certainly don't fondly remember the bullying, or the time they got an "F" in gym, or any of a million other things we all remember (and wish we could forget) about HS, but to them, the overriding thing they remember was the sheer joy of not having a plan for life and furthermore not NEEDING one, all they had to do was go though a few simple motions every day and life would be a weekday afternoon for ever and ever and ever
That's what they're trying to get back, even when I think deep inside they realize it's gone forever.
So they do the next best thing, reminisce about it with a mixture of rosy hindsight and get frustrated every time reality intrudes.....