Sherlock wanted Anderson as assistant

 (Sherlock Meta by Ivy Blossom)

There are lots of things that occur to me far later than they should. The thing about Mrs Hudson that I should have noticed but didn't was the first one: I'm sure there will be a million more, because I am not particularly detail-oriented, to be honest. This is the second thing I just realized: Sherlock didn't originally want John to be his assistant. He wanted Anderson to play that role.

Sherlock: Who's on forensics?
Lestrade: Anderson.
Sherlock: Anderson won't work with me.

Not Anderson is an idiot, but Anderson won't work with me. Sherlock then proceeds to take John along with him to a crime scene, and is remarkably patient and accommodating with him. Why does he do that? Sherlock is neither patient nor accommodating as a general rule. When John deduces only the most obvious and basic details about Jennifer Wilson's death, Sherlock isn't scornful at all. Why does he behave this way?

Lestrade: He's not your assistant.
Sherlock: I need an assistant!

We know he doesn't bring John along for his own innate skills, because when John asks why he's there, as they're standing over Jennifer Wilson's body, Sherlock says, "Proving a point". He doesn't really care what John has to offer to the investigation of the crime scene, that's why he's not bothered by John's simple description of the cause of death. That wasn't the point.

He's trying to make Anderson jealous.

These two appear to have a tumultuous relationship; they must have argued over bodies before. Sherlock must respect Anderson's abilities; why else would he want him as an assistant? So Sherlock wants to demonstrate, in the most childish way possible, that if Anderson won't play ball, he will lock him out altogether. That's the worst thing Sherlock can imagine: being tantalizingly close to the crime scene and his own dazzling deductions, but not be able to hear them or participate.

So Sherlock slams the door in Anderson's face. It's not because noting that Rache is a German word is dumb (It was something that passed through Sherlock's mind as well, as we know); it's because Sherlock is proving his point.

I don't need you. See? See me not needing you? How do you like that, Anderson? HOW DO YOU LIKE THAT? I can deduce the fact that you're cheating on your wife with Sally, I know what you were doing last night. See? Aren't I good? That's how good I am. You want to be my assistant, Anderson, don't you. You need to, because I'm dazzling and you need to watch me. If you ask really nicely I might let you act as my assistant next time. I just might. 

Sherlock doesn't realize at that point that the perfect assistant for him is John. Anderson won't fire a bullet with the steadiest of hands through a window for him. Anderson won't help him behave appropriately in social situations and smooth out the rough edges of his interactions. Anderson may admire Sherlock's abilities, but he'd never say so out loud. Anderson will never love him. John's arrival utterly alters the job description of "assistant" for Sherlock. We never hear the word "assistant" again, in fact (to my knowledge).

Sherlock's reactions toward Anderson, and all the negative things he has to say about him thereafter, appear, in this light, to be more a reaction to Anderson's rejection of Sherlock than any scornfulness about his abilities.

Sherlock has the emotional life of a nine year old.