After seeing Sherlock summon the courage to tell John his feelings on the runway, only to falter at the last moment, Mycroft realizes it's never going to happen as long as John and Mary are together. He comes up with a plan to drive John and Mary apart and clear the way for his baby brother...by having an affair with Mary.

+Mycroft falls in love with Mary despite his best efforts not to get "involved."
++Mary is actually working for Moriarty and going along with the affair to try to play Mycroft for state secrets.