He sighs happily. It's perfect—just enough pain to feel really good, not enough that he has to work to keep liking it.

(Sherlock has almost never had to work to keep liking it, except at the very beginning. But he's seen Tony's face, heard him struggle to describe what it's like, and he understands it the way he always understands Tony. He knows every step on the road from here to a whispered Sherlock, and the how and the why of each one.)