As a child, Sherlock was sensitive and a little emotionally immature, but other than that, he was healthy (not normal, but he was healthy). But his father/parents/family got tired of him acting younger than he was, and told him to stop crying, stop caring, stop feeling so much, because after all he's a boy so he should act like it.

So Sherlock froze himself up. He taught himself to be cold and detached. Unfortunately, no one told him that when he fell in love, it would thaw him out and ruin years of hard work to become 'heartless'. (His partner loves him no matter what, though :).)