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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My guess is, to make him more mystorious like the game already is aka the campaign. Either that or he ran out of battery life.