I would have to say that it is fairly inevitable.
First, discussion usually just winds up as a two-sided disagreement. People don’t walk into most threads or most discussions wanting to question their outlook on the world, and challange their convictions. So, the discussion escalates (deteriorates?) until it becomes a gestalt. Apples or Oranges? A or B? No grey area whatsoever.
Plus, the internet is a perfect for tool for anonymously taking out frustration and hostility. There are no repercussions, and there is very little threat of being physically harmed as a result for your actions.
As somene else mentioned, we are all on the same footing, here, as well. No one’s taller, or stronger, or has more facial hair. We can’t see if anyone’s scared, or intimidated. So, essentially, there’s no reason to back down.
As someone else mentioned above, it’s pretty much inevitable that if you start up a political discussion, there is going to be at least one doper out there who completely disagrees with you, and who had a bad day at work, so their just going to lay into you.
A real challange would be starting a thread, and asking people not to be intentionally sarcastic or mocking. Impossible, because truth, and the beholder, and all that. But, interesting.
All in all, though. I wouldn’t expect people to treat anyone oline any nice than they would treat someone in person. I would assume that all of you would rather nice people in person, and wouldn’t be half as disagreeable or argumentative as you on the the SDMB.