Why was Six afraid of Seven?

Because 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 Lucky's. But the boys are gone now... he knows that. It's.....it's just that he forgets sometimes. And sometimes the way that she (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.
Six's wife tried to tell him a good story the other day. Six doesn't remember the last time he heard a good story he could laugh at.... She couldn't understand why Six couldn't relate. Nobody seems to understand the toll it takes on your mind... Seeing all those innocent people die by your hands.... But we had orders... We had to do what we were told.... We... We had to.... right?? But that day... That day she looked at him with those eyes... That look of helplessness.... The same way Charlie looked at him before he got... No.. No... It's too much... The way they all looked me just before it happened..... That look of terror.... The look of fear... It was the same look of desperation she was giving me right then... And it was terrifying...