John’s face, he was sure Sherlock knew.  it didn’t even cross him mind that Sherlock didn’t know he was his best friend 

Like look how sad his eyes get when he has to say yes. And then he says it a couple times “of course you are of course…” And then to be sure, says it in words “You’re my best friend.” So there is no confusion, so Sherlock hears him say it don’t look at me I need a moment