It took two days for Sherlock Holmes to fall in love with John Watson. All it took was a word of praise, for John to say, "Brilliant!" without even thinking. Sherlock had loved him in that moment. When John killed for him –shot a man because he might be in danger – Sherlock would have done anything to make him happy, even for the smallest instant.
Sherlock didn't understand 'romance', he barely understood people beyond motives and facts. But he remembered John's face when he had "cured" his limp, a small gesture in exchange for the compliments. He knew correct manners, even if he chose to ignore them (unnecessary, a waste of time and brain power). But he knew that when given a gift, one gave another in return. Later, Sherlock's gifts to John were never things that he could hold, but moments and smiles, gifts that would last. When he saw John standing calmly at the crime scene after the cab driver was shot, Sherlock's brilliant mind knew exactly what had happened. The great Sherlock Holmes, the man always told he was heartless, discovered he had a heart.