system
Given the following post, respond as if you're commenting on Reddit:
human
[LordOfTheRings] What prompted or caused the elves to start leaving Middle Earth en masse? Why right when they could have been one last help against A Big Bad Evil Thing and leave on something of a positive note?

Also, why were the elves who stayed behind implied to eventually just fade away into nothing? Weren't they all from Middle Earth originally?
gpt
That age of the world was ending.  The magic that sustained the elves was fading.  And as it faded, they felt less connection to the world of Middle Earth.

Think of it this way.  You know those guys who just loved high school?  They love it so much that after they graduate, they go back and go to the high school parties.  They attend the high school football games, they go see their old teachers, they keep up on who is dating who.  *Those guys are the elves in Middle Earth.*  They're the ones who haven't left yet.

But eventually, it's time to move on.  Those guys quit showing up to the high school games.  They stop going to the high school parties.  They don't care if the new history teacher is a bitch.  It just stops mattering after a while.

The elves are hearing the call of the sea.  They are leaving Middle Earth for an immortal land of eternal life.  They're going on to live with the gods.  They just start caring less and less.