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The four times Sherlock failed to set up Mycroft and Greg, and the one time he succeeded.
Sherlock can see that Mycroft is starting to break under the strain of his work and loneliness. His plan? To set up a friendship between his brother and Greg Lestrade and maybe turn it into a relationship.
Mycroft comes to bail Sherlock out and meets on Gregory Lestrade. Sherlock is a little too intrigued by the way the D.I. brings a smile to his brothers face and repeats the experiment a few times.... for science.