Why is kindness taken for weakness?

Why do agressive people think they can take advantage of nice people?  Yeah sure, I'm nice most of the time but when I'm rubbed the wrong way,  I slowly distance myself from that person, eventually to a point where I dont see them nor talk to them anymore. If I work with them... I'm forced to make snappy comments everytime they come at me the wrong way. It kills me how "surprised" they are with my reaction to their action. I dont act out and argue with the person because it's just easier for me to not be associated.    For the most part, I meet great people all the time but every so often, a "Miserable" will try me.  Why do you have to be mean to people to "gain respect" or is it ?