Love
As the Fates wove the story for what would be the first of countless times, they knew it had to have staying power, and the greatest thing to stick in the minds of gods and mortals alike was love.
This would be a love story for the ages despite its outcome—Orpheus's selfless love for Eurydice that drove him to Hades and back, and the desire to see her once more overriding all else as he turned. It was a tragic, timeless story from the start.
As Hermes tried for a happier ending, the tapestry's love deepened and grew richer. Now his own love for Orpheus as a surrogate father was woven in, the sorrow when he failed tempered with the hope that one day he wouldn't.
Eurydice's love for Orpheus changed her whole worldview, showing there was more to living than just surviving, and they understood and forgave each other when the cold drove her away.
And Hades and Persephone's ancient love, once fraying at the seams, slowly healed over the course of Orpheus's journey. By the end they were dancing like newlyweds in spring, and every time, Hermes hoped this would be it.
Unfortunately, the Fates themselves loved a good tragedy.