Now that I know what I'm without
You can't just leave me
Breathe into me and make me real
Bring me to life
Leaving John for the very first time had hurt. It had also made him realize what John was to him, how much he needed him.
Without John, the great Sherlock Holmes is nothing but an addict, except that this time around, he doesn't settle for marijuana, cocaine, or heroin, he goes for the pure, deadly pain of neverending torture at the hands of whatever remained of Moriarty's cobweb of lies and death.
Coming back to John - coming back cooking for John -, and seeing him with Mary fucking Morstan hurt even worse.
So he does everything to provoke a reaction out of John, doing his best to be an insufferable prick. So when John finally snaps and punches him and they go down together, there's a short moment between time where their half-hard dicks brush against each other.
John is back to keeping his distance in seconds, it doesn't even take Mary pulling him off Sherlock for that.
He wishes they would've kissed.
(They never did, and they never will. John marries Mary and knocks her up and Sherlock is the first to leave after the ceremony. And then Mary Morstan shoots him, and he dies. Except he doesn't - he's Sherlock Holmes, after all, and this isn't the first time he comes back from the dead; the truth is, he only comes back because of John - for John.)