I once had the opportunity to summit the Villarica volcano in Chile and peer into its mouth. The mountain felt alive, expelling pulses of hot air like it was breathing out of its fiery gut. It evoked such mystery and splendor! Who was this massive soul who breathed fire? It was easy to understand how the Incas could build a theology around such a real, natural phenomenon operating on a much grander plane than us piddly humans. For all anybody could tell, the mountain had obviously been there forever and always would be with its searing heart.
From the post "Why does faith = redemption?", theists and atheists seem to agree that our respective belief systems are built on underlying assumptions. Given an initial framework, life becomes a cycle of world view -> behavior -> world's response -> adapting our world view -> adapting our behavior, etc. Sounds like evolution to me! It also offers a simplistic explanation for the creation of any belief system.
One of the classic Richard Dawkins quotes is, "We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further." Why is it so easy for religious folk of a particular faith to see the splinter in the eye of other religions, but not the log in their own? (And here’s a bone for the theists) What trees are in the atheist’s eyes?
I would say that it is due to history and culture. Any belief system, even atheism, is subject to seeing what we want to see. Because assumptions are all tied together, starting with some basis, reinforcing and expanding it is rather straightforward. Opposing it takes a whole lot of work. Indeed, human nature seems to direct us to become "set in our ways". We become self-protective and defensive when we see a threat to our cultures and ideologies. The young earth creationist, for example, recognizing the threat of evolution to his assumptions is likely to turn a blind ear and give the knee-jerk response that all the information necessary to understand creation is in the Bible.
What brings us to blows or at least not to see eye to eye (with all that wood we’ve go in there!) is the subjective nature of the argument. Given the same embodiment of our opponent, the same genetic make-up, thinking ability, family, and history, would we not have the ideology and behavior of our opponent?