Reading your post, I wanted to verbally fight with you, only for a second though, but it's not my experience with a majority of posts.
Most of the time I react like an explorer, looking for whatever's missing and trying to find out the hidden agenda or point of view. Then once I've found it I search for my own opinions or knowledge that would best build apon what I've found and then write it in the most constructive way possible.
I don't think all forum posters are fighters at heart, but more so that all ego centric conversations are a fight in themselves. Like you said, there are views to defend, opinions to express and arguments to poke holes in. Have you noticed that sometimes after some conversations you are mentally tired? It is quite like you have literally been in a battle, fighting. Any time someone strongly expresses a view or opinion as "The way it is" there's the instant wanting to attack that stance if your views or opinions differ. The difference of opinions and wanting to be right is what causes the conflict and essence of fighting in the conversation.
I see that a way out of it is to catch yourself as you are posting a fighting post, and then see where you can add to the discussion. Instead of tearing down the other person's views, opinions or facts, find a way to build a new view of things that is positive and hasn't been expressed before.
I think the best way through is to be aware of yourself and your actions and go from there.