There is a fundamental difference between ‘I could love you, but I don’t’ and ‘I don’t love you, but I could’. That is the crux of the difference between Tyson’s thoughts about John Watson and Sherlock Holmes. The type of love differs, but the point remains.
Tyson absolutely does love John. They are simpatico, brothers-in-arms, brothers by all but blood. Romantically? Oh, Tyson knows that he could have fallen for John, easily, but he doesn’t love him romantically. And that’s fine. Whatever alternative path they could have taken, Tyson is happy where he stands with John.
Sherlock, on the other hand, is a more complicated feeling for Tyson. He does not like Holmes, but Tyson knows that he has a paper thin defence against caring for him; and that defence is avoiding him like the plague. He purposefully keeps a distance from the detective, because he does not need the Moriarty Mirrors game to become any more complicated by fallnig for another genius determinded to burn themselves up.