He's always hungry. Always, always. He eats and eats (at least when he's allowed) but it's not fair, as soon as the food goes into his mouth it goes away into air and nothing reaches his stomach at all.
That's why he likes to eat people, when Lust lets him. People go away into air too, but there's always a little left, a little warm red glow that trickles down inside him and stays and made him a little less hungry.
She promised him that someday he would have special food, food that was all warm glow and nothing else, and when he eats that, he'll finally be full.
He hasn't found the special food yet.
But he's still looking.