Note the grin. Poor John.
John grins like that when he’s angry or uncomfortable. Here, clearly, he’s trying to act natural. He’s trying to be as ordinary as possible to hide what’s actually going on.
Sherlock is on his left, talking to Janine. Hello there, strangers. Here I am, a normal person just like you, holding my best friend’s coffee for him, nothing strange going on here. We’re supposed to be here, obviously.
I’ve been dreaming about him. The psychosomatic tremor in my hand, the one he cured, it’s come back. I’ve been thinking about leaving my wife. I’ve missed him so much, and he’s gone and replaced me.
He’s talking to his girlfriend. He has a girlfriend now, you know. It’s madness, isn’t it? A girlfriend? My Sherlock? He doesn’t do girlfriends. Or boyfriends, as far as I know. I thought he was gay. A GIRLFRIEND. It doesn’t make any sense. It was only him and me until now. Now there’s her.
Nothing strange going on, nothing at all. Walk on by.
He can swallow all those feelings for now, but he’ll fail once he sees Sherlock standing there with an engagement ring. Then it all rushes to his face.