Joan is happy, knowing the effort Sherlock is putting into being friendly. When it comes to Ted, in Sherlock's defense he had not expected visitors when he first met Ted, and Ted was taking the attention and presence of Sherlock's partner and best friend. Keeley and Roy were expected and he isn't going to lose Watson to them.

Sherlock would have gone on, but the doorbell rings and he rises. "I shall return," he says, heading to the front door. Joan watches him go, smiling fondly.