Nature
If there was one thing Hades knew, like breathing, it was human nature. Whether it was something the Fates, in their wisdom, had woven into his being, or something he'd picked up over millennia of observing death in all its forms, he intimately knew the idiosyncrasies of mortals, all their coping mechanisms, good and bad.
And what was human nature was also godly nature, at least as far as he knew. Why else would his beloved wife turn to drinks and drugs to cope, why else would Hermes take in a poet boy out of kindness, despite his words?
And why else, Hades reflected, would he spend his waking hours petrified that one day Persephone would leave him and never come back? He made his citizens work harder to chase the thoughts out of his mind, drowning himself in work and gathering souls.
But amid the machinery and paperwork the anxiety only grew, and he wondered, as mortals did, why human nature was so deeply flawed.