It's been quite a while now since you were orphaned. Both your parents, found murdered in some alley. That was how you met Sherlock. He'd solved the murder, of course. That's what he does.
After this, you became curious about the man. Who wouldn't, after all? After some time of following his website and every single headline that related to him, one thing became very clear. His thought process was remarkably similar to yours.
Eventually, you were inspired to contact the police with your own insights, but you were simply not taken seriously. You were "too young" for anybody to really believe you knew what you were doing. Anyone except Sherlock.
You tried tagging along with cases, but of course John initially wouldn't allow you. "Too dangerous", he claimed. Sherlock, however, did not care in the slightest, and brought you along anyway.
Now, you are incredibly close. He's like a brother to you. You have moved into the upper apartment, and life seems to be good!
One day, Sherlock comes to find you. Rather than being greeted with a "Hello!" or a "How are you?", you are greeted by a file being unceremoniously slammed down on the table in front of you. How sweet.