A.N- I can explain. And yes, I have seen the YouTube video.
There had been a particularly challenging series of cases for Scotland Yard, and now, at last they were over (for the moment at least). And so, the team (meaning Lestrade) had found it appropriate to host a private celebratory party, as they hadn't done so in a while in felt like they deserved it.
This is why currently the scene is of a rather drunken Police Force, a lot of loud music and a certain Army Doctor and Consulting Detective, dancing together in the middle of the floor.
And it was then that the booming tones of the previous song ended, only to be replaced by the familiar and slightly acoustic notes of the beginning of the well-known song, Carly Rae Jeperson's 'Call Me Maybe'.
John and Sherlock continued to (not quite soberly) dance to the song, and as the lyrics went on, Sherlock (typically) felt the need to pedantically point out that-
'You can't miss someone before you've met them- they were never there to miss,'
-As his analytical mind had heard the lyrics-
Before you came into my life, I missed you so bad-
And, as Sherlock said this, John huffed a sigh at the detective, rolling his eyes.
'What?' Sherlock asked, a little irritably. 'Was it something I said?'
'Sherlock,' John stated- 'Just dance with me.' And, as he said these words, Sherlock wrapped his arms around the smaller man- who returned the embrace. And, soon enough, they were dancing nose-to-nose, locked in each other's arms, wishing to be that way forever.