A couple of things. First, things change. Even in places where it doesn't feel like things change, they do. So if you leave a place and come back it will be different.

More importantly, we don't look at an objective past. Our minds remember the best and the worst. So when you get older you remember "the good old days". Those days, objectively never really existed. They were just days. So when you're 40 you won't be able to recreate the magic of being 21, or that feeling you had when you went home and someone was cooking your favorite meal, or go back to your hometown and feel the way you did when you and your buddies hung out.

I'm probably explaining it poorly, but it boils down to nostalgia being a hell of a drug. You never know when you're living in the good old days until they're gone.

Luckily it works in reverse to an extent. If you had a really shitty childhood, you can look back on it and say "at least it's not like that anymore!" The psychological damage is already done, but you're not coming home to an alcoholic berating you or heading to school to a teacher beating your ass ever day.

You can never go home again, both because things have changed and because that place only ever existed in memory, and the real world was some amount (generally GREATLY) different than what we remember.