It's because of Tony, of course. Must be. Sherlock said as much before - what Tony loves, what makes Tony happy, gains his own affection. Now that he's satisfied himself that Obadiah's presence in Tony's life is a good thing, he loves him.

Obadiah wonders if Sherlock has feelings of his own, or if everything is a reflection of or response to Tony.

"As long as it doesn't bother you," he says.