Because people who have no hopes are easy to control

Gmork: Foolish boy. Don’t you know anything about Fantasia? It’s the world of human fantasy. Every part, every creature of it, is a piece of the dreams and hopes of mankind. Therefore, it has no boundaries.
Atreyu: But why is Fantasia dying, then?
Gmork: Because people have begun to lose their hopes and forget their dreams. So the Nothing grows stronger.
Atreyu: What is the Nothing?
Gmork: It’s the emptiness that’s left. It’s like a despair, destroying this world. And I have been trying to help it.
Atreyu: But why?
Gmork: Because people who have no hopes are easy to control; and whoever has the control… has the power

via Tiffany Imogen