Sometimes he finds himself staring at her photograph, wondering how would it be if he ever allowed himself to fall in love.
Not that he's ever experienced any sort of attraction towards a woman; the late Irene Adler being the only notable exception, but he doubts that his feelings for her actually resemble what ordinary people call 'love'.
He admired her for her brains rather than her beauty, and he still respects her memory; death took her away too soon, he won't get the chance to put his wits against her anymore.
Death is definitely the greatest enigma of all.