The modern “you”
Most of the argument is about the singular “they,” but the singular “you” also has an interesting genesis, and this article in The Economist tells the story, or at least part of the story.
The article concludes that social change has brought about this language change, though it does not draw any conclusions about why social change, meaning egalitarianism, did not also destroy the familiar “tu” in French, Spanish, and Italian, and the familiar “du” in German. What was it about English social change that was tied so tightly to language? That’s just something to think about.