Does he have PTVD caused by some horrifying experience in battle or even something involving family deaths? Or do you think he just prefers not to talk? If so, then why does he prefer not to talk? Anti-Social? Let's hear some possibilities. Be creative.
[Edited on 08.29.2011 10:29 AM PDT]
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I think there may have been an incident in training or something where he might have been the only survivor. Maybe its part of his focus, he has a job to do, he's going to do it.