Usually because there passionate about what they believe, and they have to be right. They don't like to be wrong. We live in a society that teach us to be competitive, someone must win, and be defeated. You can see this in politics, religion, video games, sports, jobs, everything we do in this world tells us to be a success and if we don't we lose. Instead of teaching people everyone wins, or both you and I might be right, and just share what may work from both sides, our world just rather have that winner at all costs.
No one wants to fail, because when you do, everyone looks down on you. When you care what other people think of you, most people will internalize it and think they are bad, wrong, or have failed.
Those that have learned to value their self worth, and not care so much about whether the win or lose, are not so much offended.