Why is kindness a responsibility?

I’ve grown up with “be nice to others!” “treat others the way you want to be treated!” and it’s not like I minded it; somehow it just made sense. Of course you should be nice to others!
But I’m getting to a point where I’m looking at people, and some of them really get to me. I so disagree with the way they view life, the way they behave, the way they treat others.
And still, I hear “you have to be nice to them.” Like it’s my responsibility, my job, to be kind to people regardless of what they’re like.
I hate that! Why shouldn’t I get a choice in how I treat someone? Why should I give everyone the same treatment when they’re clearly not the same person?
...What are your thoughts? Do you treat everyone kindly regardless? What are your thoughts on how one “should” interact with others?