An existence theorem for true love

Regarding Carl Sagan and his relationship with Ann Druyan, from her chapter about Carl at the end of the book Billions & Billions:

Lynda had observed that the sustained incandescence of our love exerted a kind of tyranny on those around us who have been less fortunate in their search for a soul mate. However, instead of resenting our relationship, Lynda cherished it as a mathematician would an existence theorem, something that demonstrates a thing is possible. She used to call me Miss Bliss. Carl and I especially treasured those times we spent with her, laughing, talking far into the night about science, philosophy, gossip, popular culture, everything. Now this woman who had soared with us, who had been with me on the goddy day I picked our my wedding gown, was there our side as we said good-bye forever. For days and nights, Sasha and I had taken turns whispering into Carl’s ear. Sasha told him how much she loved him and all the ways that she would find in her life to honor him. “Brave man, wonderful life,” I said to him over and over. “Well done. With pride and joy in our love, I let you go. Without fear.”