It used to be that, generally speaking, particularly in what was referred to as "polite society," people kept their opinions to themselves. It was considered good etiquette to avoid incendiary subjects; we've probably all heard the old adage that it's best to avoid discussing politics and religion. I think this idea was on its way to being pitched out the window during the civil rights and women's rights movements, and then on into the environmental movement, when we saw a rise in tribalism and identification with causes. When "all politics is local" became the cry of special interest groups, the need to assert one's adherence to ideologies became quite important to some. (Ever thought about when bumper stickers really took off? It was in the '60s and '70s, for sure.)
In his book Seculosity, David Zahn presents the thought that other social scientists have observed: as society has moved away from the formerly religious underpinings of gathering and belonging, individuals instinctively search for something to which to belong. We find our people in political parties, social causes, obsessive attention to body image or fashion or home remodeling/house flipping or whatever seems to ignite our passions for getting with other people who have the same ones. Along the way, we make sure everyone knows our passions and we have given ourselves permission to press them upon all passersby.
Thus, I cannot go for a run without seeing a sign in the window of a home proclaiming, "In this house, we . . . " with a detailed list indicating the owners' politics. As I run by, I think, "Hmmm. Wonder why they think it is important that I know that." I don't know them. They don't know me. What compels them to make sure everyone driving by knows what is believed inside? I could understand a religious proclamation. But this is the same to me as listing foods to be avoided or the best breeds of dog: purely personal preferences.
Certainly the rise of online communication, including the ability to comment on articles and blog posts and tweet our opinions on anything and everything, has turned us to a practice of always making sure everyone knows our thinking on every subject at any given time, and insisting that if others do not agree, they are scum. When we see this practiced all the time, we begin to fall into it ourselves.
Here's the thing: it's a mark of self-regulation to keep our opinions to ourselves. I don't need to know what you think to work with you. I can't read your mind, so don't help me with that if you know it could cause conflict. You don't have to like me to treat me with respect. It's none of your business how I vote, what I read, and the beliefs to which I subscribe, and I don't need to know the same things about you. None of us knows what another thinks unless the other opens their mouth to tell us. Zip it. It really is best to avoid some topics, and it is simply one of the timeless principles of good behavior to keep opinions to ourselves and get along.