Why was 6 afraid of 7?

Six hasn't been the same since he left Vietnam. He can barely close his eyes without opening them again at the fear of Charlies lurking in the jungle trees. Not that you could see the bastards, mind you. They were swift, and they knew their way around the jungle like nothing else. He remembers the looks on the boys' faces as he walked into the village and... oh Jesus. The memories rarely left him. Sometimes he'd reminisce- even hear - Tex's southern drawl. He remembers the smell of Brooklyn cigarettes like nothing else. He always kept a pack of Lucky's with him. The boys are gone now. He knows that; it's just that he forgets, sometimes. And, every now and then, the way that Seven looks at him with avid concern in his eyes... it makes him think. Sets him on edge. Makes him feel like he's back there... back in the jungle. He always sleeps with one eye open, and packs a loaded 9mm under his pillow.