[A smirk. Sherlock's every waking moment seemed to be dedicated in inflicting as much sarcasm as he could at any poor soul who had the misfortune of coming across his path, so he was well-versed in picking it up. Plus with being around the likes of the sass master John Watson...]

I can only assume that there is some probably some sort of terrible overarching reason behind all of this. Probably a terrible one.

[He's delighted at the thought of an incredible mystery. Despite his reservations and disbelief, he's suddenly quite cheered up.]