As iron sharpens iron, so a man sharpens the wit of his friend. (Proverbs 27:17, JPS)This verse is usually quoted to point out the value of friendly debate, sharpening our wits and our knowledge. So why aren't we all that sharp?
If the verse is about debate, then our wits could be sharper if we took the time to debate each other as with a friend. With a friend, we are patient. We would never assume bad faith or stupidity or moral failing on their part, just because we had not succeeded in communicating our point or convinced them we were right. But those things are common in disagreements with strangers.
Sometimes we talk about controversies when no one who disagrees is present. Whether we intend it or not, this works out to discussing the other group behind their backs, and only when we are safe from contradiction.
Sometimes we simply avoid arguments. It's too easy for disagreements to become hostile. And so begins the separation into different groups who distrust each other, who avoid each other.
Within the Christian faith, denominations are sometimes in this type of situation. Each group quietly avoids the other, or criticizes in private where no one will contradict. And typically no one checks the accuracy of what is said about the other group; the "facts" being discussed probably become less accurate as time goes by without any dissenting voice to keep people honest. Isolation is bad for perspective. And if "iron sharpens iron," then denominations should not avoid each other as we do.
One thing the proverb makes clear: Our failure to talk to each other is our own loss, even if we're right. I'll say it again: even if we really are right, and the people who disagree really are wrong, still we lose something for not talking to other people -- because both sides gain from talking to the other, as iron sharpens iron. If we skip a conversation because we think we have nothing to gain by it, we've expressed contempt for the other person. If we are so dismissive of others that we assume that we have nothing to learn from talking to them, then we are wrong about that, regardless of whether we are "right" about the point of disagreement.