"Her mortal lover died, and her daughter, Persephone, disappeared. She blames mortals for her pain." So, love? Grief? Misplaced blame?

And it says a lot about Thanatos's current emotional state (or lack of one) when he follows up blankly with "I can only be at one place at a time. No one is dying." Hermes and Charon cannot bring shades to Hades if the very act of dying is not present.