Sherlock somewhat embarrassedly tell John that “somebody” personalised their text alert noise on his phone for a joke, but it stays that way for months. Even though he looks mortified every time it goes off, Sherlock never takes the ten seconds or so to simply change the text alert noise back to something else. Why not? Because it’s turning him on, on some level. And as John remarked in his blog (over Sherlock’s Heroic BSOD over Irene’s ‘death’) it’s highly likely that Sherlock, having had pretty much no experience in this kind of thing before, doesn’t really understand what he’s feeling every time he hears the text alert noise. Only that it feels kind of .. good.