"Quick! If you love me!"
Holmes says that as if it's a given. As if there is no doubt in his mind that Watson loves him. He says it as if he could stake his life on it (which, in this case, he kind of does). He doesn't just believe it, he believes in it. It's something he can rely on.
Whichever way you want to swing it (in friendship or romance), it's beautiful.













