After hearing what Mycroft said to him while drugged during HLV, Sherlock decides to put Mycroft in that state again and see what he does. He has the mixture adjusted to put Mycroft into a similar state without actually knocking him out and slips it into his tea during a normal conversation.

Confused and completely without a filter, Mycroft becomes really affectionate, letting it slip that he has been lonely without Sherlock around, trying to hug him like when they were children, etc.

And then Sherlock discovers that Drugged!Mycroft is willing to do pretty much anything his little brother wants if he thinks it will make him happy...