Dexter really wants someone to approve of his murders and be proud of him because of how well he does them! Pat him on the head and tell him he's a good boy and he's fully prepared to kill for you!

It is interesting as Dexter's feelings start showing up more and he does an uncalculated murder. Dexter is an unreliable narrator about certain things, especially his own emotional state, and I think the show actually captures that well. (Also that's way more interesting than "This person genuinely doesn't feel anything.")

Dexter on Person of Interest would be messy, yeah. He'd be easily won over, but "I am definitely going to do my own murders on the side" would be a problem.