There's a scene where Sprite monologues to his fellow Eternal Makkari about their immortality. Makkari's been in love with Sersi for forever. Makkari doesn't even remember this, but Sprite decides to fill him in -- oh hell, here's the passage:
"Oh, Makkari. I loved listening to you telling your story. The hardest bit was keeping a straight face. I mean, just the idea of you being all Ooh I love you over Sersi.This made me feel how lucky we are that we get to die. I know, it's a funny sentence, but there's a truth to it.
"You two broke up about half a million years ago because she was cheating on you. Then you broke up about twenty-five thousand years ago because you were cheating on her.
"You broke up about sixteen hundred years ago because you were bored with each other."
We have a time limit to our lives, and we need to make as much of that time as we can. When that time comes, we have to face it, and whether we do so happily or not is what matters. The Eternals aren't so lucky, so time never really means anything to them. They can break up and get back together and waste all that time in their lives, and it won't ever matter to them.
Well, unless you're Sprite. It must suck to be Forever Young.