What was John to Sherlock?
This difficult question had kept him pondering for hours.
Was John his lover?
Yes, they shared a bed, and had for months now. But that was only during the night. During the day John's attitude went back to 'concerned flatmate' and hardly ever lover. Not that Sherlock minded this arrangement; he was more than happy to keep his work with John far away from their escapades at nights.
Was John his soldier?
John is the only man who was willing to chase after him, no matter what the danger. More than once he has put his own life on the line, expecting nothing at all in return. Always near to hold Sherlock's hand and tell him that, truly, everything will be alright. The perfect soldier to help Sherlock fight himself.
Was John his anything?
Did he really have anything to do with John other than helping him pay the rent?
Finally, he snapped out of his pondering to see that it was well past midnight, and John had already gone to bed. He watched John sleeping silently, barely moving but for the rise and fall of his chest.
As he slipped into their bed he finally realized what John was to him. John was the thing that kept him childishly selfish. John was his teddy bear.