why do so many discussions turn contentious?

@nonoise said "[and]Bob's your uncle."
I love that expression; a colleague used it many many years ago and I eventually found the alleged source for it. You know it (perhaps apocryphal)?

I don't know  the answer other than that I suspect these fora aren't moderated heavily (unless a post or discussion is brought to Tammy's attention- sometimes, I get the sense she's doing it all). And there are forums that are far more heavily moderated and  don't tolerate nonsense, politics, etc. You get banned. So, folks tend to behave nicely in such an environment. And that doesn't stop informed discussion of the relevant topics- audio, music and associated products, methods of implementation, etc. 
Self-policing is always best. 
A little humor is also usually welcomed (at least by me). But I saw one "newbie" say in another thread that he was reluctant to post because of the rancor and nastiness. I've gotten to the point where I just ignore it. It's not my site, I try to follow the rules and treat others civilly. Beyond that, I don't think it is possible to avoid disagreements; argument can be at a fairly high level without resorting to personal attacks. And there is learning there, even if you don't buy into the original poster's premises.
As I learned a long time ago in the law, "the most reasonable person" often wins. Posting here isn't about "winning" but a lot of the contentiousness seems to arise from arguments about the merits (or lack of them) to a particular piece of gear or approach. And I think no matter how passionate a person is in advocating a position, they must take account of how their post will be read by others.
Being provocative to garner attention isn't really a good way to build relationships, either on the web or in non-virtual life.  

@mijostyn: Actually I'm not Neville Chamberlain but thanks for the thought. I was, starting in the mid-90s, dealing with Internet appropriation of proprietary matter, and pretty quickly learned more than a little about behavior on the Net. If you consider my approach to be "politically correct," so be it. I'm not fighting anyone, and am not at war. Perhaps that is part of the problem- one of perception. As I said earlier, I have no solution to this-- I'd like to think moderation is unnecessary, but the reality proves otherwise. 

I hope I'm not an angry old man. Getting old, but in the process, seem to have less and less to get angry about, at least as relates to the topics relevant to this forum. 

@mitch2 - fair point. I took a break for a year or so from this forum b/c I found the signal to noise ratio deteriorated.
But what are my options, other than reporting what I consider to be an obnoxious post? I don't usually do that for several reasons. I see threads that are larded with political discussions and do just ignore them. 
Other than participating in good faith and encouraging same, what else can one do? I don't run this board, and think part of it really does have to do with the lack of moderation (with apologies to Tammy who has enough to do).
Beyond that, I think you are right, though, it isn't just what one tolerates, but if the rules aren't applied people are pretty quick to cross the line. Plus, what I may find offensive or stupid may be just peachy to others-- thus, I begin to question why I'm the one still here. (No, I'm not trying to suggest I'm leaving-- if I do it will be without fanfare--).

Apropos the "technically informed," I think the key is exposure. You have to have seat time; all the engineering including audio or circuit design knowledge in the world will not translate fully into what you hear in a given system. If it is good, you want to know why. If it isn’t, same question. I think the value of having a lot of input from users who have spent time with equipment, given their stated controls, may be more valuable than measurements or the theoretical opinion of someone knowledgeable in the art but at a remove.
That is largely the value of this forum as far as I am concerned. (There is also troubleshooting, not really limiting what value there may be to others).
This place isn’t necessarily where one goes for deep on music, although there are a couple of long running threads and the occasional post; instead, it’s mostly gear centric, given the nature of the business to which it is attached. (kinda obvious).

Apart from known synergies with commonly used (don’t read: cheap) equipment, there are a lot of questions lurking in the interaction among components, something that seems to be addressed almost by happenstance.
I’m actually glad for it, because from my vantage point, I have yet to see a "whole picture" (holistic) philosophy in home audio that is satisfying to all. And if we are spending time here to learn and improve the reproduction quality of an audio system, that time ought to be well-spent.
I guess point 1 is that sometimes, it isn’t solely the province of the engineers, though they are vitally necessary and 2, the real key is to hear stuff in the wild, put in place some controls if you are considering outlaying real money (whatever that is for you) toward a component, and have some experience with the limits of what a good reproduction system is capable of, before signing off on a chosen direction.
I’ve found that choices made a long the way often dictate other choices toward a certain end result. Perhaps a different way of saying "everything matters," but to me, each component has some coloration or character that you are matching with other things to result in the overall sonic outcome from the system. Those choices may pre-determine (depending on your objective) the next step in many cases. (Apart from wholesale changes to entire systems, which are costly, at least at the higher end of the scale). It’s impossible to speak about this in the abstract, but it is the area where I think the forum has the most value. (to me)