High School and being a teenager for many is really the last chapter of youth. The last time you and your friends can just hang out without a care in the world. After that, people tend to start moving away, caring more about actually doing something with their life... ...and then, you really start to appreciate those years, once they are over. The summers spent playing minigolf, or going to big loud concerts in the fields at night... your first kiss, perhaps, or the first time you got drunk.
...then, like summer itself, it just dies, and you need to grow up. It all gives way to another group of friends and moments that you simply need to catch on to. The past is gone and can never be relived, but it will reincarnate itself as the new friends and memories you get.
This kind of sad death of a lifestyle has a lot of examples in both reality and fiction.
For some, the Summer of '69 is considered a very literal spin on the subject, where the youth actively tried to keep their youth, and some of them were killed for it, leaving their friends to grow up without them and become today's old people. Other fine examples of this however can be found in fiction...
Zelda: Ocarina of Time. Link makes several friends as a kid, and then when he sees them again as a teenager, he systematically waves goodbye to each one of them, as they go become sages. However, he didn't spend very much time with any of them to begin with, except Saria. (And when it comes to her, Shiek does discuss this sad subject with Link.)
Rockman: Omoide no Okkusenman. This entire song, set to Doctor Wily's first stage from Mega Man 2, is all about this very subject. A Japanese office worker reflects on his childhood, and the friends he left behind, wishing he could have it all back. He is unaware though that new friends are waiting for him, including a girl who likes him.
Chrono Trigger. Our very own beloved RPG's epilogue is entitled "To good friends", in reference to the fact that now everyone must return home. The memories and the adventures together were tedious before, but in retrospect... we'll miss them. we'll miss them all.
(Chrono Cross pumps this sort of thing into the introduction.)
The Archie sonic comics might do this too...
