"Yup." Sora nods. "If you get turned into a Heartless, you can only come back if your heart and body can find their way back together again. Otherwise, you get lost in the darkness, trying to find lights to eat, and lights are in other people's hearts." And that's why Heartless find other people and eat their hearts. "It's scary. By the time my world fell, there were only a few worlds left, and I think all they were was lucky. There wasn't a way to, y'know, predict it. Unless you had a Keyblade, there wasn't really a way to stop it, either."