Why do people always feel like they can take advantage of me?

I'm one of those genuinely nice people, but I wouldn't necessarily call myself a push over. I have no problem defending myself or standing up for myself. Butt it seems like most people want me to take it to that outrage personality. I greet everyone with a nice positive attitude. I respect everyone and I am friendly with all until I sense or feel like they are trying to get one over on me to prove themselves. I'm not no one's tester nor lab test. I like being nice and I don't like when people test me.

I feel like nobody ever truly like me. It's like people are always saying some negative about me behind my back even when I'm minding my own business. Why do they feel the need to walk all over me, and expect me not to do anything about it?

How do I change my personality? I'm not naive nor insecure. How do I appear more like a threat? I'm 26 years old female 5'6 172 lbs. I don't want a physical altercation, I just want people not to think twice before testing me. I want respect.