Fate
Clotho spun, Lachesis measured, and Atropos cut. The tapestry they wove was volatile, ever shifting to accommodate both the changing decades and Hermes' myriad attempts to change the story. It was hard work, weaving for eternity and singing in the back of hearts and minds, and they counted themselves lucky there were three of them.
Hermes' determination was taxing, though never frustrating; the closest he came to annoying them was calling them old. And as long as the messenger god tried to divert fate, they would be there to keep the tale on the path it was meant to follow.
The sisters did not consider themselves unkind, though fate, as they all knew, could be cruel indeed. This was simply how things were, had been, and would be, and destiny was as inflexible as Hades himself.
At the very least, the sisters allowed Orpheus and Eurydice a full summer of warmth and love before winter. Fate, they knew, wasn't always cruel.