Why is kindness often viewed as a sign of naïveté?

I try to be kind, upbeat, etc in my interactions with other people, especially at work. I want to treat others well, work together peacefully, and leave the project happy. I’ve noticed that some people (particularly more curmudgeonly folks) seem to read that attitude as naïveté, and feel like they can walk all over me or treat me like I don’t know what I’m talking about. Why is that?