Being a nice person, and being a genuine or honest person are not naturally at odds. I can't vouch for the motivations of everyone, but I've gotten the "Nice Guy" label before, and can at least speak for myself in this regard.
When I go out of my way to do something for someone else, like hold a door open for someone, or cover them when they're a few cents short at a cash register, people might thank me, joke that I'm being chivalrous, or whatever. Though appreciation always feels good, I have other reasons for doing these sorts of things; they're near-effortless actions I can take, which generally save someone else considerably more work. Acts like that help create a tiny boost of efficiency, and make the greater machine of society, and the lives of those who make that society up, that much smoother. It's the basic spirit behind civilization itself. Every big thing, like a social environment, a national industry, or a person's lifetime, are each made up of individual choices and actions taken by the person or people that make it up, and everyone that interacts with them... and then, everyone who interacts with those tangentially-connected people, and so on, and so on.
So yeah, when I compliment someone's attitude, admire their ability to tell a joke, or thank them for helping me with something, it's because I'm impressed by or appreciate that quality they have... and I want them to cherish that aspect of themselves, because I feel it's making a positive difference in the world. This same spirit inspires critiques too... saying things like "You're a fantastic cook!" or "Learn how to drive!" both factor into this philosophy of leaving the places, people, and things I encounter better off than they were when I found them.
I think, when people are encouraging children to be nice to others, it's because they lack the language to be able to express how their every action contributes to the big picture. We, and every choice we make, connects us to everyone and everything else in thousands of ways we will never be able to know... and that's not a spirituality thing. It's a causality thing.