We tend to make up our minds for emotional reasons. Then we identify with our conclusions, and find ourselves threatened or angry when someone tries to alter our conclusions. The more comfort we derive from our belief-based conclusion, the more fiercely we resist someone trying to change it.
Hence all the hostility amongst religions, or competing scientific theories.
We should be, at the very least, polite in our arguments -- and always have good, solid supporting evidence.
For politeness: How to Argue on the Internet and Linguistic Politeness Research Group
For good, solid arguments :