Sherlock knows he's handsome. He fusses with his hair far longer than he admits, but John notices. He dresses in a way to highlight his best assets - long toned neck, narrow waist. John's seen him use his face to endear him to suspects, to catch people off-guard.
As good as he looks when he's dressed in a nice suit, when he's making an effort, there's something about him when he's genuinely relaxed, traipsing about in one of his dressing gowns, laughing at a joke John's made, getting absorbed in research. He's not trying, it's effortless, and John's the only one who gets to see that. The way his hair rumples on one side when he sleeps, the way his eyes and the bridge of his nose crinkle when he laughs, the way his robe hangs off one shoulder when he's distracted.
John stares at him, studying Sherlock's profile, limned with the warm light of the setting sun. Those soft brows, that upturned nose, those lips. He's lost in thought, studying something on the screen of his laptop, when his eyes light up, his mouth forming a perfect round O. He's always handsome, John thinks again, but it's the moments like this - unawares of his surroundings, not putting on airs, caught up in something, that his attractiveness is nearly blinding.