Honestly, I don't even blame people who get mad at the suggestion that they should be nicer. I don't begrudge the "you're trying to get something over me by advocating for kindness" response when I see it.
Like, it's one thing to hear that from a place where you haven't experienced kindness (or at least, haven't experienced kindness consciously because the kind and caring acts directed at you have been background radiation against the noise pollution of the world being Bad) because you don't know it's power, then. I don't think it's unreasonable, if your only experience with kindness is being a sucker for wanting to be good to someone (because they asked, or because you were attached to them).
It's an entirely different thing to hear it after you've gone through instances of people being good to you when you didn't deserve it, or when someone thanks you for having been good to them when you didn't even realise you were doing it, or when you are good to someone because you feel like you should be, fully expecting it to go to hell and then having it not do that.
I get it. It's like how people say you can't truly love another unless you learn to love yourself (perhaps better phrased as "learn to be loved") -- you have to have experienced kindness to know what it does and why it's good in practice. Considering just how selfish and emotionally illiterate we all start out (and we really do -- and anyone who argues that they don't must have been a fucking nightmare as a kid) of course kindness looks like being a sucker until you've experienced it first hand.
It is an immature and enraging response to get when you just want people to get over themselves and show a little maturity, but I get it. Growing up fucking sucks, even if after you've done it the world is a lot less bleak.