From “Republican Like Me: How I Left the Liberal Bubble and Learned to Love the Right,” by Ken Stern, former CEO of NPR and lifelong Democrat (excerpt published in the Daily Beast):

We are becoming angrier and more polarized not because of increasing issue disagreement but because we are increasingly participating in groupthink. When all the people you know, when all the people in your political sect agree with you, it becomes easy to relax in the certainty that you and your cohort are right, and the other side is not just wrong, but also taking a long, slow bubble-bath in the sea of craziness. When we don’t know the other side, when we don’t hear from them, when we don’t talk to them, when we can demonize them to our heart’s content, there are just no brakes on our sense of self-righteousness. Polarization is increasing because polarization is increasingly easy.