If there's one thing Sherlock Holmes hates more than anything else, it's stupid people asking stupid questions.
"Sorry, got to dash, have to pick up my riding crop in the mortuary."
The normal, boring and stupidly arrogant reply to this statement is an incredulous, 'Why's your riding crop in the mortuary?' to which the proper answer is a witty remark and a 'Good day to you.'
To be fair, it is a strange statement to make, even more so for the world's only consulting detective; in usual circumstances, he'd have forgone this announcement in favour of blessed silence and a quicker exit from the room. It's a tedious practice, really, announcing one's plans and he personally finds it a rather boring, callous and stupidly arrogant waste of words by the normal people of the world.
In truth, this statement is more of a test than anything else, because, there is nothing he hates more than stupid questions asked by stupid people, and this 'John Watson' doesn't seem like a stupid person. It is only when the remark is met with John merely shrugging it away with a slight bat of an eyelid that Sherlock becomes truly intrigued. This ready, unhesitant acceptance is soon followed by a very much not-stupid question, and this, above everything else, is what makes Sherlock turn back.