I have a bit of a thing for swearing loyalty, and secret crushes, and who doesn't like just a little bit of power imbalance. During wartime Lord?/Prince?/General? Sherlock has had his eye on one Captain Watson and when one John Watson is captured Sherlock says he'll kill him if John doesn't swear fealty, the thing is Sherlock really, really wants John to swear himself, even if he has to seduce him into it. After all, his old commander left him to be captured, and left to his death, surely even Sherlock could take care of John better than that.