Since I don't know you, and I assume you don't know me, the way you have pointed your language at me demands that I challenge you to a duel.

"Thee/thy/thou" was the familiar or informal, while "you" was formal. You would use the informal with someone who was close to you, a friend or a romantic interest. You would also use the informal as an insult to people you didn't know well, as a kind of "I'm better than you, so I can use this intimate form to refer to you" thing.

Ultimately, "you" became standard in all cases, because it avoided the possibility of unintended insult. In similar fashion, I think we'll end up using "singular they" a lot more often, because of the rapidly increasing awareness about gender fluidity and wanting to avoid unintended insult, just like with "you."