Interestingly enough, "thee', "thine", "thou", etc. are the informal of you (present), your, and you (past). Equivalent to "tu" in spanish instead of "usted".
The reasoning is that "thee" is proper for usage with intimates, and the Lord should be your intimate.
That's true.
The point is, I don't talk to the Lord the same way I talk to my high school buddy.