why do so many discussions turn contentious?

I've heard differing reports, but the internet as it once existed was dismantled in 2016-17.  Now 97 to 99% of what you see posted is AI bots "conversing".  The people who programmed them have lost control and the bots have taken over.  I am not a bot, but that's exactly what a bot would say.

I have always been impressed by the amount of knowledge and passion for audio and music expressed in these forums.  I like how people are willing to share their experiences with others.  This is true when I first joined (2000?) and it's still true today.  I think the OP's question is a little Pollyanna-ish in that there's always been heated discussions, sometimes respectful and courteous, but frequently childish and mean spirited.  It's never been perfect.

What encourages me about this thread is the number of people who have contributed who have low posting counts.  Is it possible that there's a vast silent majority of forum users?  I have 4,000 some odd posts, but it's taken me twenty years to reach that total.  My post are always highly intelligent, insightful, technically astute, historically accurate and built on a foundation of unassailable logic, but even I wonder if I really had 4,000+ things to say about audio and music.  Yet I've also noticed that some user(s) have multiples of my total posts, but have only been posting for two years.  Is it actually possible for someone to have 15,000 things to say that are worth saying within two years?  The obvious answer is no.  The easiest solution for dealing with people who really don't have anything worthy to contribute is to just ignore them.  You see their names multiple times in a thread -- don't read the thread.  In the long run you'll be happier.