It happens at precisely 3:49 PM, on Tuesday, the sixteenth of April. He is sitting on the couch, bored out of his mind. John has gone out to do the grocery shopping. By Sherlock's calculation it usually only takes him seven minutes to get there, thirteen minutes to find the milk and bread and debate over any microwavable meals that he thinks he can make Sherlock eat. It then takes around seven to ten minutes to bag everything and pay at the checkout, depending on how difficult the chip-and-pin machine is being that day. It then takes another seven minutes to come home, which he should have done already. However, John Watson is exactly twenty-six minutes late, and Sherlock Holmes is not impressed.
And then it happens. A dull, throbbing sort of ache in his temples. It doesn't hurt enough to distract him from the vast amount of nothing that he's doing, but it's annoying and it won't leave him alone. He decides to nickname the strange sensation Anderson, and leaves it be.
Exactly eleven and a half minutes later, John comes home. He's late because he was having another row with the chip-and-pin machine, but walked away with a discount and a date with the lovely shop assistant. Strangely, the ache gets worse.
It happens again exactly three and a half weeks later. He is dragging John through a construction site, looking for clues on their latest case; the not-so-accidental death of one of the men who was working on the site. The scaffolding he and John are trying to navigate is less than stable, and John falls through.
A few hours later, while Sherlock paces the waiting room and scares all the nurses by snapping at them and making rather cruel deductions, Lestrade frowns at him and tells him to shut up, sit down, and stop stressing about John because he'll be fine.
It hits Sherlock then, and he marvels at the sudden realisation. He's been stressing about John to the point where it's affecting his physical wellbeing, and he's sure that if he lets it go on, it'll affect his ability to solve cases. And, really - solving cases and making deductions is all he has. He has to do something about it.
Abruptly, he stands up and walks past the nurses, who move out of his way with quiet squeaks of horror. He can hear Lestrade calling after him, asking where he thinks he's going, but he doesn't listen. He makes his way to John's room, where he finds the doctor awake and looking a little dazed.
'Sherlock,' John starts, but he's cut off.
'John.' his voice is clipped and cold, and John looks surprised. It occurs to him that he's never seen this side of the detective before. 'It's come to my attention that you accompanying me on cases is becoming more of a hindrance than a help. As I've told you before, I consider myself married to my work. Allowing obstacles to get in the way that could possibly stop me from working to my full ability are utterly out of the question.'
John is speechless for a moment. 'Sherlock, what's going on? I don't understand.'
'Once you are dismissed from hospital, you will have seven days to pack your belongings and find lodgings other than Baker Street.' Sherlock talks blankly to the wall, unable to meet John's eye. The headache is worsening and easing at the same time.
'I - you can't do that!' John looks both offended and confused. 'We're friends!'
'I don't have any friends,' the detective replies coldly, and that cuts off any protests that John has been trying to make. 'I wish you a speedy recovery, Dr Watson.'
After that, it takes John exactly three days, seven hours and thirty minutes to pack his things and say goodbye to Mrs Hudson. She seems disappointed to watch him go: after all, she did like John a lot more than the rest. But Sherlock's decision is final, and for the first time in a long time, he refuses to speak to her about the matter.
And so Sherlock throws himself back into solving cases. He ignores the snide remarks from Sally and Anderson about his "breakup". He brushes Lestrade off when he shows concern. Mycroft says nothing, simply shaking his head in disappointment. But the detective couldn't care less. What he told John was the truth: he didn't have friends, because he didn't need them. As long as there were still crimes to be solved, he was useful. He didn't need other people for that.
It isn't until a few months later, lying awake at night and staring at what used to be John's armchair that he realises the headaches haven't stopped at all.