Our high-functioning sociopath isn't a people-person, but that doesn't mean he won't try.
Sincerity
Sherlock wasn't "good" at people. He was a master of many sciences, could tell a computer programmer from his tie, and spoke fluent French, but when it came to people he just didn't have any tact.
That's not to say he wasn't a master manipulator; he'd learned that lying was a useful skill early on. He knew how to win over people's trust with shallow appearances and minimal effort role-playing.
However, he sensed that John was different, and that it would take more than a little acting. This, he realized, remembering John's strong navy blue eyes, would take sincerity.
Prompt was #88: navy