Anywho, I used to be one of those kids with little to no capacity for empathy. I wasn't horrible, I would never have hurt anyone, but I had a really hard time relating (nor was I trying to). My pre-school teacher mentioned to my parents that something was off with me (they just thought I was a weird kid), it seemed I had no notion of social feedback, putting me somewhere on the autism spectrum.
Turns out I was going completely deaf, which on the long run ended up being a blessing in disguise (also because I more or less recovered my hearing later on).
Being deaf, you learn to read people as if they came with an instruction booklet, it comes very naturally. I saw how other people had those reactions, emotions showing in their faces, their body language and so on, and how it affected others. I started feigning it and playing with it, effectively manipulating people into reacting the way I wanted them to.
As time moved on, some of these things started coming naturally and it soon hit me like a brick: the best way to get things from others is to understand where they stand emotionally and to show reciprocation and support, i.e. empathy.
It often makes me wonder whether empathy is purely an acquired trait, or if some people have a strong predisposition for it.
The one major drawback of empathy, in my opinion, is feeling utterly useless when you see other people repeatedly hurting themselves, usually thinking they're doing the right thing. It pains me, deeply, but I suppose it's a necessary side-effect.