Why is six afraid of seven?

Six hasn't been the same since he left Vietnam. Every time he closes his eyes, he's sees Charlie hiding in the darkness of the forest. Not that you could ever see those bastards, mind you. They were fast and they knew their way around the jungle. He remembers the looks on the boy's faces when they walked into that village and... oh Jesus. He shouldn't think about that now. Sometimes he still hears Tex's slow southern drawl. He remembers the smell of Brooklyn's cigarettes. He always had a pack of Luckys. But the boys are gone now... he knows that. It's--it's just that he forgets sometimes. And sometimes the way that seven looks at him... it makes him think. Sets him on edge. And he feels like he's back there... In the jungle... In the darkness.

Seven has a hook for a hand as well, which is very scary.