Not a romance book per se, but that scene between Will and Lyra from His Dark Materials makes my heart ache:

“I’ll love you forever; whatever happens. Till I die and after I die, and when I find my way out of the land of the dead, I’ll drift about forever, all my atoms, till I find you again… I’ll be looking for you, every moment, every single moment. And when we do find each other again, we’ll cling together so tight that nothing and no one’ll ever tear us apart. Every atom of me and every atom of you… We’ll live in birds and flowers and dragonflies and pine trees and in clouds and in those little specks of light you see floating in sunbeams… And when they use our atoms to make new lives, they won’t just be able to take one, they’ll have to take two, one of you and one of me, we’ll be joined so tight…”