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.
Why did Rob McElhenney gain weight ?
McElhenney gained the weight to mock the vanity of his character, Mac, and because he was sick of seeing sitcom stars get better-looking as their shows age. He has no regrets about becoming the first actor to make a "Raging Bull"-sized committment for a TV comedy. The first part of the new season, especially, makes the most of Mac's new size. Look for his character to try to cover up in Hawaiian shirts in future episodes.
Where is the front of Paddy's?
A street sign says Second Street. In Philly, numbers are used and not spelled out, so I am guessing it's somewhere else.