This is just my conjecture, but I think that it's that psychopaths lack the capacity for empathy, and empathy is otherwise like a muscle in that it can be trained. I suspect this because I've grown more empathetic compared to when I was a kid, and some other people I've talked to said it was like that for them (not very scientific, I know). I remember being somewhat selfish and amoral.
I think it's a combination of the environment you grew up in, the behavior of the people you grew up with, the values you were raised with, and the education you received, and some of it is also purely self-driven. And so toddlers and little kids are like amoral sponges, since they're still developing their senses of justice, morality, and empathy.