Disclaimer: nothing mine. I blame Kaspersky antivirus for this plot. :-) Sorry – but not sorry for the abundance of brackets.
Maintenance
It's about a month after Sherlock's move in 221B and it's time for renovation. No, not in the flat. In his Mind Palace. Many things have changed, after all. Some things must definitely be kept, and some need to go and free up space best allocated to new experiences.
He begins by defragmenting the hardware lest it become cluttered and slow him down. He deletes past addresses, saves the new one (the mind palace acquires the brass plate of his current home). He adds a room for John (a whole room, instead of the drawer or shelf memorable people usually deserve). His flatmate has the rare talent to surprise him, after all. He keeps his fan's name with a shortcut for a speedy retrieval, (he'll need it, hopefully sooner rather than later) and deletes the details of the case that gave him that information (who cares which password the last victim chose for her phone; and anyway, John blogged about it, as abysmal as his titles' choices are).
When all is sorted out, he runs the mental equivalent of an antivirus, to make sure he hasn't missed something that could become a liability. Astonishingly, he has. John. Riskware appears in bright yellow next to John's name. In fact, John is the epitome of riskware. "Software which actually was not programmed and intended as malware but has security critical functions. These functions can be used to start or stop computer processes or computer services. It can be executed or misused by malware." Not that it is easy for John to be manipulated. Mycroft can attest to that fact. But the life they live – the life they like – is dangerous. Danger is what brought them together, bound them at once, like handcuffs without a key.
John forgives what would make most people run away from the detective as quickly as possible. He killed to protect Sherlock when he barely knew him. John admires him, and Sherlock likes that. He likes it too much, probably. But, when you're trapped between Mycroft (who finds his deductions only mildly lacking and still uses Sherlock to do his work– the lazy git) and the 'normal' people (who use freak or a variation thereof, except for Lestrade who also just uses him), it is very difficult not to soak in John's praise like a sponge.
John is useful on cases, too, and not only because of his medical and general knowledge and protective streak. Sherlock connects clues with more ease when he's around. The inherent hazard is born from how easy – too easy – it would be for him to get addicted to John. Actually, Sherlock isn't sure it hasn't happened already. He wants the man to stay after all. Yes, he knows it's improbable; but even John holding out this whole month was improbable to begin with. Sherlock suspects that losing John, even this early on, even not traumatically – just because his flatmate left, which is to be expected – would jam his systems for awhile. So riskware, definitely.
Well, he has some options:
Delete?
Quarantine?
Ignore?
At last he decides. He clicks 'Ignore'. A life without risks is nothing but boring.