I just wanted to talk about this shot here for a moment.
I’ve been close-watching the show for other cues, but I noticed:
This is the only time Mary makes John laugh.
From chuckles to full-throated giggling, we get to hear John laugh quite a bit in every series. He always does it response to—or in relation to—Sherlock. (ETA: Just posted some screenshots of a few moments over here, for illustration: [x].)
Let’s take a look at the dialogue:
JOHN: As you know, these last couple of years haven’t been easy for me; and meeting you … Yeah, meeting you has been the best thing that could have possibly happened.
MARY: I agree.
MARY: I agree. I’m the best thing that could have happened to you.
JOHN: Well, no. That’s, um …
Read another way, italics mine:
JOHN: Yeah, meeting you has been the best thing that could have possibly happened [since the death of my best friend and partner].
MARY: I agree. I’m the best thing that could have [ever] happened to you [full stop].
JOHN: Well, no. That’s, um … [not exactly true].
That laugh? It’s a bit of surprise, incredulity at the audacity of being re-worded in the middle of his own proposal. I think it’s a fond laugh, and he’s sort of charmed by her confidence (it reminds him of someone he knew), but it’s a different kind of laugh than the others we’ve seen.
John doesn’t know Mary is a linguist, but she’s subtly correcting him here. She’s leading him.
Hell, Mary should have been the one stepping forward in that waltz.
And as we’ve seen, she never makes John laugh again.