why do so many discussions turn contentious?

@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--).

No, because you always get the conceited clown who feels it’s his duty to insult those who do not fall in step with his “sensibilities”. 

You always get the technically informed members who feels it's their duty not to let the "snake oil, voodoo" products run unchecked on these forums, that are making a fortune for their shyster manufacturers, that never come on here to protect their products when called out!

Self-regulation would be nice, where participants communicate theIr opinions respectively and accept that not everybody is going to share the same viewpoint.

“You always get the technically informed members …”

The ignorant always consider themselves as “technically informed”.

“who feels it’s their duty “

Nobody’s have a tremendous need of an outlet where they can pretend to be a “somebody”, deluding themselves that what they have to say has value to others, and that we actually are interested in their opinions.