Remember growing up? When you were little , if you are like most of us , when you strongly disagreed with someone you could not do so without disliking or at least being angry at them. But as you got older normal developmental processes helped you be able to disagree but not disrespect. Some people, for whatever reason, never seem to go through this developmental stage mentally or socially. Therefore when they disagree with you the insults , dislike and disrespect fly as if they were 10 years old. Because in this particular way, they are. That's my opinion anyway and it certainly seems to explain the behavior of the types you are speaking of.
Naturally after a few rounds of that we can all find our "inner child" and respond in kind out of frustration once in a while.