The dementors liked Karkaroff because he was weaker and more entertaining than the rest. The other captured Death Eaters were stoics, mostly; they screamed infrequently and never cried, adopting instead a sort of slumped hollowness, staring emptily at the walls.
Karkaroff, however, gave a good show. Pry: he whimpered. Pry deeper: he wailed. Pry deeper still: a cacophany of screams and sobs, bloody fingers tearing at the frigid stone, the sweet song of suffering ending always in broken silence.
It was not possible for dementors to know disappointment, but they came close when Karkaroff left. He was such a delight.