I can see where it comes from easily. You try appealing to someone's reason and it fails. You try appealing to someone's empathy and it fails. You're arguing against a closed-minded heartless brick wall of an opponent; you're not really going to convince them of anything, in any way, because their purpose in life is to disagree with you no matter what. So the best you can do is hope to dissuade others from falling in with their same lot, and one way to do that is to just publicly look down on the opponent, snub them, make them seem like the uncool kid who nobody wants to be like. So you get smug. Your opponent is an obvious moron and you publicly can't see any reason why anyone with two brain cells to rub together would so much as give them the time of day. They're not even worth listening to. Hopefully, if you give off that air, people won't listen to them, and won't fall into the trap out of which they can't be reasoned. Of course if someone asks what makes them so uncool, you still give reasons, but the default before that conversation happens is just an air of them not even being worth talking about.