Disclaimer: Premise and characters belong to Steven Moffat, Mark Gatiss, and, of course the incredible Sir Arthur Conan Doyle, I'm only borrowing them for a bit of non-profit fun.
Note: Can't not think of a reunion scene in this fandom.
Emotions are not, have never been, Sherlock's area. But unlike the solar system they did have impact his work and so he'd made a careful study of them. Still, despite his studies, the physiological tells for emotions were never as precise as the calluses on a person's hand or the tan lines on their skin. Sherlock has never wished for a better intuitive grasp of emotion more than he does now.
John's lips are compressed in a tight, hard line; indicative of anger. Sherlock had anticipated this possibility. John generally expressed a need to 'get some air' when angry with Sherlock; the hand, locked like a vice around Sherlock's bicep, says John doesn't want space this time.
Sherlock talks because he doesn't what else to do. He explains why. He apologizes. Then there's so much he'd wanted to tell John while he was gone, it spills out to fill the space that's come between them. "I missed you."
"You had a choice."
"Do you realize how difficult it is to fake a death in this day and age? But as long as you believed no one looked closer. You protected us all. If they'd realized they would have killed you… Mrs. Hudson, Lestrade… I would be dead."
The grip on his arm softens but remains. A sigh, "At least you're back."