I mean it’s kinda the real life tragedy of love exaggerated, innit? Irl people die young or one person dies old and another person dies even older. At the end of it all someone gets left behind and has to learn how to move on after that. And for the one who dies you know you’re leaving them behind. You know you’re dooming them to moving on and if you believe in an afterlife god only knows how long you’ll be waiting for them on the other side. The tragedy of the immortal loving the mortal takes those feelings we all know about and rips your heart out about it.
Telling a couple of versions of that story right now. It's a bitch to write, and one of the best things to inflict on people on the other side of the storytelling wall.
(see also the summary of The Door Into Sunset: "the final sacrifice... will confront him with a choice more terrible than anything he could have imagined: death with those he loves, or immortality without them."