John was unattainable.
Sherlock had figured this out quickly. First off, the ex-army doctor was, as far as he could tell, straighter than an arrow. It was likely he was bi. It was worse when he got stupid, boring Sarah. John, no matter how adrenaline seeking he was, needed a bit of dullness in his life. He wouldn't even break up with the girl. He was too much of a gentlemen, even after their relationship went go stale.
Then they broke up. And the two of them kissed thanks to a speeding cabbie.
And suddenly John wasn't so unattainable.