He is lavish. Masterful. Intellectual, inexplicable, overt.
Sherlock Holmes defines extravagance. He isn't exactly the best thing for John's tireless, bland stability. John works the nine to five and tries not to lose sleep over the small things - like the labor of his fiancée's expectations, and the sirens that fill his head with noise after curfew. He finds comfort in art, and drinking. Besides the war making its presence known, slowly creeping its black tendrils into the heart of Bristol - John's life is quiet.
Given that, he's not surprised when he is dragged into Sherlock Holmes's orbit. There is no manual to help him navigate the moment their eyes first meet; he presents himself as the perfect distraction, a beautiful incendiary miracle. John thinks that maybe he was hoping for this, because: Sherlock is potent, and inflammatory, and perfectly willing to make John's life hell for his own self-indulgent pleasure. With one touch, John's sense of self is corrupted.
And what does it matter if the world implodes?
It was never kind to them to begin with.