all i am (is what you see of me)
They weren't supposed to feel anything positive.
Dementors only caused misery wherever they went. They sucked all the happiness out of humans, animals, even other dementors who risked allowing such emotions to surface.
They were incapable of all but gloom and doom.
Right?
That was what all the humans thought, anyway.
But they were wrong.
Dementors craved companionship too. Little dementors would glide around together while their mothers communicated, not unlike the way the humans did. And they usually mated for love, rather than simply increasing the dementor population.
The humans didn't see this. They called them foul beings, with no concept of anything other than despair.
This was why the dementors chose to wreak havoc on them. They couldn't change the humans' minds, so why not just give them what they wanted?