Kindness comes not easily - or naturally - to some, ["kindness is not in some people's nature", he means by that last part] nor is it necessary or even encouraged in all situations. I have heard, from people more inclined to kindness than me, that extending it to those who deserve it the least is the hardest, but often the most necessary.
I would call that a skill, then, but perhaps it is rather a matter of heart.
["A matter of nature; something one can't learn."]