He is undesirable but not unwanted. There's a hairsbreadth pitfall there, deeper and darker than one might imagine. It's tempting, sometimes, to slip down into it, pretend that he can bridge the difference between the two with a single body, his own. The very idea is fallacy, of course. Nonetheless, sometimes he likes to explore it, probe; it's the same satisfaction of an ulcer in the mouth, stinging, present, perverse.
Caring is not an advantage.
I will burn the heart right out of you.
Tears are a solution but only of salts. Irritate the mucous membranes and bodily fluids result. Grit, germs, cascades of proteins, setting off the coded paths of inflammation. Engulfment, purgation. John is a doctor; he knows about these things. Sherlock is an irritant, and he knows John knows that too.
He looks away from the gap between what is and what might be. He knows what falling feels like, after all.