Possessiveness. Not one of Sherlock’s finer traits.
John is very possessive too, but in a quiet way. He just puts a steady hand on the small of Sherlock’s back, a gesture that fiercely says ‘This one is mine. Back off.’ And he smiles while doing it.
While Sherlock snaps at people, he barks at them and when he feels that nothing helps he just drags John away.
And he doesn’t even show most of it to the world, hides the creature within that starts howling ‘MINE’ and spits and wants to kill others for touching John or for just being nice to him, always fearing that someone will manage to take him away.
Sometimes he catches John looking at other people. That hurts. So he offers harsh remarks and awful truth about those until John closes his eyes and huffs. The icy atmosphere afterwards Sherlock can live with.
Sometimes John gets fed up and angry. He asks if Sherlock doesn’t trust him. If he can’t see that his heart belongs only to Sherlock. That looking doesn’t mean anything. Because it really doesn’t.
Sherlock wants to ask ‘Why do you even need to look? Am I not enough?’ He never does though.
It’s an issue they’ll never resolve. Until it’ll happen to Sherlock. And won’t that be a surprise for our boy?