Noah Smith puts it well:
most of our arguments are over things like Obamacare, or antipoverty programs, or financial regulation-- issues on which reasonable people can and do disagree. If you’re uncivil in this sort of situation -- if you call your opponent an idiot, or a liar, or a nastier name simply because you think his or her argument is bad -- you’re basically being overconfident. You’re assuming that there’s essentially no chance that you’re in the wrong, so it’s in the public interest for you to rail against your opponent and score points with the crowd. If you do this, there’s no chance that you yourself will learn anything from the encounter.