James Moriarty was never very good at diction and enunciating his words. When he was in school, teachers always encouraged him that "if he would just work a little bit harder" it would come to him. He took lessons with a vocal coach, even became good enough to get the lead in a play, but it always came back when he was nervous. If he didn't have a script in front of him, or his words carefully planned out, he would misspeak.
When he was sitting with Sherlock at Baker Street, everything came to a head. He just couldn't think on his feet quite as fast when the sharp-tongued detective was in the room.
As he was leaving 221B, he tried to tell himself that Sherlock had understood. Even to his inner voice this sounded like a lie. Jim was trying to tell Sherlock that he owned him.
James Moriarty was desperately in love with Sherlock Holmes, and wanted nothing more than that love returned.