why do so many discussions turn contentious?

Look around, you will find your answer. Look at all the perfectly fine discussions ruined by people who insist on taking them off topic in order to discuss whatever is on their mind, without any regard for the original topic let alone the original poster.

Scroll down the page of recent discussions, it is actually pretty good right now but over time you will find discussions with the same names over and over again. No matter what the subject they will take it over. They will take a discussion started to talk about how great something is, and turn it into a discussion about how awful it is, and how terrible the people are for liking it. If you like something and they don’t happen to like you then you are a shill, and you are insulted every way imaginable. If you hear something they don’t, or understand something they don’t, or say anything at all they disagree with on any subject they will hound you off the platform. Happens over and over again.

They will blame contentious topics. They will project blame everywhere but where it lies: with themselves.

And As IF By Magic and by way of deflecting attention away ... LoL!

it takes two to argue, so my approach will always be to avoid the dance when you sense who you are dealing with; what I find disappointing is that this childish behavior largely dilutes the value of the form, making it difficult, at times, to find what you are looking for. A question or observation is posted, it leads to a thread of 50 responses, mostly of little or no value, and often leading totally away from the original post...this should  not a male pissing contest, "my X is better or cost more than your Y"....I could care less about that....but, I guess this is the world we find ourselves in, and I have nowhere else to go....

Speaking of over and over again

Moderators of online forums are almost always volunteers with jobs,families, and no time to babysit this place day and night. Tammy keeps hanging in there and probably does have to do it all at times as other volunteers come and go.

+1 nonoise and mike-in-nc  and I really miss almarg too