Mycroft's motivations have always fascinated me. You have to wonder what he's put up with over the years to develop such a protective but dysfunctional relationship with Sherlock...
For so long, Mycroft had worried about his brother. Not so much that he'd be alone forever - Sherlock could have handled that, and been content. More that he'd eventually find someone who caught his interest, but also someone who couldn't deal with Sherlock's way of life. It seemed impossible that anyone could.
When Sherlock burst into Mycroft's office, virtually vibrating with excitement, he shifted into high alert. Listening to his brother's eager ramblings about some invalided army doctor who he'd decided to move in with, Mycroft found himself subconsciously preparing for a number of scenarios. Surely, something would scare this level-headed, reasonable man off, and Mycroft would have to pick up the pieces, yet again.
He threw himself into research, learning everything he could about Dr. Watson's military and medical histories, eventually arranging a proper face-to-face with him. What he encountered amazed him, though he didn't show it. Not only was the man already fantastically loyal, it seemed that if anything, he was better off having spent some time with Sherlock. They truly seemed to complement each other, which was a far better result than Mycroft could have anticipated.
He smiled to himself. It seemed that not only had Sherlock found someone who could keep up with him, challenge him even, but who also had him completely and utterly bewitched.