No, you don’t understand.
It’s just the Two Of Us. Us.
Sherlock is including himself in a set, with John. Just, John. This is revolutionary, because before this, he did not. It was “I am” and “I will” and “I want”. If John was lucky, it would be “John, do this” or “John will follow me”. It was “Yes, aren’t they strange” and “God, what is it like in your tiny little brains!”
He never before said “Us”, or “We”. It was always “You” and “I”, and as intimate as the latter sounds, it has nothing on the former. Nothing.
And John might not get it at first, but he’ll get it eventually, and yeah, he’s as furious as I am at having had to wait three fucking years for his life to get back on track, but it will be worth it. Because Sherlock’s eyes are lifeless and his cheeks are sallow and he sometimes flinches like he can’t help it, but he accepts without question that he and John are on the same side. Maybe that’ll be worth it.