Each side considers the positions embraced by the other to be not just foolhardy or ill-advised but fundamentally unjust (“just plain wrong”) and therefore beyond politics, beyond negotiation, beyond compromise or accommodation, and so also worthy of indignation and denunciation.
How did we get here? Many people blame our increasingly segmented culture, in which media companies benefit from hyping certain forms of perceived injustice. Fox News, following the example of right-wing talk radio, does this more effectively than anyone.
Social media goes further, allowing us to construct a reality defined by distinct visions of the world: We pick who we “friend” and who we follow, creating virtual communities of the likeminded. Before social media, you lived out your days in the real world of neighborhoods and workplaces that in many cases had some diversity of views, and also uncertainty about views, which led people to conceal and restrain them out of a concern for manners.