I'm a happy introvert. Well, happy in the sense that I'm not depressed.
I did look at the cognitive functions aspect as well.
Also, since there's such an argument over his type, he is one of the fictional characters that can't be easily typed.
I hate to use myself as an example, but it's the first thing that comes to mind.
I've taken the Cognitive processes test three times. I've taken practically every credibly MBTI test out there multiple times (the real one first though). I'm typed INTJ for a very large percent of the MBTI tests, and I've been typed INTP only when I rush it. (I've tried experimenting different tests with different mindsets and the like)
However, on the cognitive processes test, I scored INTP, ENTP, and INTJ respectively each time I took it. (over like two weeks)
I have mostly Te but very close Ne, Ti, and Ni. And yet I know personally (from social steryotypes to trademarked descriptins to my own insight) that I am INTJ. So maybe there's something similar going on there. (I'm definitely not an expert and so I have little to suggest here).
Also, as was pointed out, he is in constant pain. it is touched upon that he is "happier" on stronger meds, less pain, etc. -both as a moral issue in the plot line and development of his character. He is also extremely angered when the truth is kept from him (Lol, season 5 episode 17- watched it last night. free on some site) and would be much "happier" with the truth. The pain may play apart in changing what we see to what he "truly" is without it.