What is the difference between being defensive and being angry?

I was thinking about a couple times at work when people told me that I was being defensive. Usually, this situation would occur when someone was making, what I considered, to be a really poor suggestion and I thought it a waste of time to even have to make a counterpoint against it. (I.e. I was thinking, “you’re an ass.”). I’m not saying it’s nice to behave this way, but curious as to why this behavior is labeled “defensive” and not simply “impolite.”