The last time Sherlock had laid eyes on Jim Moriarty was after John had told him to leave Molly alone when he had gotten a little too drunk and a little too hands on. Sherlock didn't blame Moriarty for backing off and avoiding them – John could be forceful when he wanted – but he didn't think that he would be brave enough to seek them out.
They sat in the hospital cafeteria because even after all this time Sherlock doubted that John would want to see Moriarty and for what seemed like hours they sat in silence; until Moriarty made a comment on a case that New Scotland Yard had solved with Sherlock's help and Sherlock suddenly remembered that he'd actually liked Moriarty during High School. He had been the only other person in his year that actually challenged him.
They chatted for a while until Moriarty's phone beeped and he announced he had another appointment to get to. Sherlock walked back to John's room alone but the moment that John looked up Sherlock knew that he hadn't been as descreate as he had originally thought.
'So, what did Jim Moriarty want?' John asked looking back down at the book on his lap.
Sherlock didn't even bat an eyelash or think about what he was saying before he replied: 'Just to boast.'