Because man, I love me some awkward mornings-after. I can never have too many of them.
John looks across the bed, the tangle of sheets and limbs - his own tanned, muscular ones and Sherlock's long pale ones - and groans.
"Sherlock, was this..."
"Nope."
"Would you let me finish?"
"Nope." Sherlock, prosaic as always, interrupts John before he continues. "You're worried we've made a terrible mistake. You're worried we've ruined our friendship, that we won't be able to go back to what we had. You're worried I'm going to regret this, or get bored. Is that it?"
John's mouth opens and closes, rather like a fish. "Um... pretty much, yeah."
"Well stop worrying. We live and work together. We rely entirely on each other, we trust each other more than anyone else. This was the most logical progression."
"...Logical. Thanks. That's what I've always wanted to hear from someone I've shagged. This is never going to work."
"John, you forget one thing. You've said it yourself - I'm incredibly stubborn. I want this to work more than I've wanted nearly anything in recent memory." He's silent for a moment, giving John time to process that statement. He just stares at Sherlock. "I'm committed to making this work, John. I'll do whatever you need me to do. Be whatever you need me to be."
John smiles, finally. "Silly git. You're already who I need you to be."