I wonder what the difference is, I mean beyond just painting someone who can't as a jerk.
I think it has to do with learned skills of emotional self-regulation... someone with those skills is better able to hold their perspective while allowing that of another. But how does one learn that?
It might have to do with safety. I suspect that the people that are better able to have those kind of civil discussions have had good behavior modeled to them in the past, that has increased their own sense of safety, like from someone else in a position of power that has demonstrably respected their opinion even when they held different values.
Could that be the krux of it, that most "uncivil behavior" in this realm is from people that are reacting from a sense of danger, like that if they meet a person with differing values they automatically believe they are being threatened? (Recognizing that sometimes that belief is correct.)
I also wonder if there's a universal central reason behind why it is important to understand someone else's argument if their conclusion differs than yours.