why do so many discussions turn contentious?

One of the reasons is posters that refer to anyone that disagrees with them as 'sick', full of 'malignant narcissism', and 'not sane'.

Some are over passionate but your observation is well taken.  
The faceless peril even occurs in SMS to friends.  
Misconstruence propagates.

I blame"@your least favorite poster here"

Well, builder3 is just showing his cards based on previous encounters that didn’t go his way, proving my point, and taking it personally.

jw944ts,
almarg was a sage compared to the rest of us here. Gentlemanly in all ways, always seeking to find a middle ground to resolve any contentiousness. He’d go out of his way to see both sides and even go so far as to try to talk sense and to reconsider their stance by logic and reason, which are long lost arts.
He was a devout and steeped audio enthusiast and very well educated (he was a patent attorney, among other things) and the most even keeled person I’ve met here. He never took offense.
Some of the people I alluded to took exception to him and treated him disrespectfully, which got a lot of our attention, and sadly, marked the path this site has taken.

All the best,
Nonoise

@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.