Sherlock wasn't clever.

Well, he was, but he was more than that. Being clever is just knowing facts, recalling them; a parrot could do that. What Sherlock had was something far more precious, though less valued by most: intelligence.

In fact, he had it in bucketfuls. Which was why, when he first saw John Watson, he instantly knew quite a lot about him. All the obvious stuff was available through mere cleverness, but intelligence could tell him something else entirely. To be precise, three things.

1) John Watson was similar to Sherlock himself in that he, too, was bored;
2) He had a quiet, mule like stubbornness about him that could prove troublesome;
And 3) He most probably wouldn't object to the violin being played at half three in the morning.

On the basis of these three facts, Sherlock decided that John would make an excellent flat mate, so long as those hideous jumpers were kept firmly to his side of the flat.