Sherlock's eyes went wide, and for a moment he stiffened, but just as quickly he relaxed into the kiss, just in time for John to let him go after a nibble on the lips.

Sherlock stood there for a few seconds, eyes wide, panting slightly, looking at John. "That's... certainly an interesting way to say hello." He finally managed to stammer.

"You.. are upset, because you have to leave soon." Sherlock deduced, though that was easy to see in his face. "And you... don't want to leave London." Or maybe John didn't want to leave him? Sherlock really wasn't quite sure.