Why Six is Afraid of Seven

Why was 6 afraid of 7?

Seven’s been worried about six even since he left Afghanistan. Every time 6 closes his eyes, he sees the war and hears the gunshots. He sees the blood, the killing, the death, and soldiers falling. When he looks at seven, he remembers when they were forced to eat their own flesh to not starve in those caves. He sees the war and the flashbacks will come back forever, burned into his soul and mind.