Sherlock lost John as a partner because when he got married with Mary and had his daughter he wanted to lead a normal life (but they were still friends). At John's wedding, Sherlock met one of John's sisters. Emma, an ex-police officer, lived in Glasgow and planned to move to London. The only one among the guests who didn't seem to be an idiot. So Sherlock told her to be flatmates and she accepted. John didn't like this at all but well. Not out of jealousy, but out of protection towards his sister. The fact is that Sherlock strangely felt attracted to her so he did some strange things sometimes. But Emma didn't fucking pay attention to him.