One stops aging after the age of 20, and starts again after meeting their soulmate. The majority find their soulmates quickly, and live up to 100, but there's that little percentage who don't find and live up to 1000. Yerin wants to be in the majority. Yerin wants to believe that Yuna is her soulmate.
Immortality is amazing.
You get to watch humanity advance and decline, you don't have to worry about disease, and you don't have to feel attached to anything in the world. Why wouldn't anyone want this?
That's what I thought up until I met Yuna.
Now, immortality is the one thing I never want.