For me a big part of it comes from frustration that I (we, science) know the answer to things, and someone refuses to accept it.
We don't know everything of course. And most of the time the excuses use are illogical, paradoxical, or easily disproven, and yet the person refuses to understand that theatre wrong. Hence frustration.
I never feel frustrated if my child doesn't understand something or refuses to, because she's a child. But treating adults like children is condescending, hence frustration.
> The other side welcomes everyone and presents their ideas politely.
This is a huge problem. It's just like how people selling snake oil are fantastic listeners and make people feel welcomed and understood, just to sell them literal water disguised as medicine.
We are hardwired to trust nice people over logic of facts :/