The show has never come out and said this, but it has been a running joke and strongly implied that Dennis is in fact a sociopath and the rest of the gang simply doesn't realize it. In the episode "Mac Fights Gay Marriage", Dennis says "This is crazy, I'm having *feelings* again. Like a 14 year old kid or something. You remember feelings right?". Mac, shocked says "Are you saying you don't have feelings?!" to which Dennis replies "Well no, I'm saying I've built up a shell. A cold, calculating, heartless shell..." Dennis is also shown to be a complete narcissist with no regard for anyone else and thinks of himself as a "golden god" and he often implies that he has disturbing sexual fetishes.
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The show has never come out and said this, but it has been a running joke and strongly implied that Dennis is in fact a sociopath and the rest of the gang simply doesn't realize it. In the episode "Mac Fights Gay Marriage", Dennis says "This is crazy, I'm having *feelings* again. Like a 14 year old kid or something. You remember feelings right?". Mac, shocked says "Are you saying you don't have feelings?!" to which Dennis replies "Well no, I'm saying I've built up a shell. A cold, calculating, heartless shell..." Dennis is also shown to be a complete narcissist with no regard for anyone else and thinks of himself as a "golden god" and he often implies that he has disturbing sexual fetishes.