why do so many discussions turn contentious?

Our lack of social evolution also tied much to our physical natures, testosterone and alpha wars.

Humans are clever and inventive, thus humankind evolves in the technical sense. Socially, we've likely not evolved more than five minutes. Someday, if we're clever enough may invent robots who'd be rid of us.
Also, for far too many, this human life on earth is consciously temporal, the perfect afterlife is just around the corner, why waist your time trying to make this mortal plane a better place.

Humans have always been opinionated, I'd suggest opinion may be becoming an obsolete descriptor as it pertains to ideas. New modes of information technology has inculcated self belief in the masses of expert status in whatever field they choose. An idea pops in your head, you find information to support it (seems humans prefer confirmation bias), easy and intentional to ignore information refuting or questioning it, ever more information supporting original idea, repeat ad nauseam. You now have created a believer, belief is the same as objective truth to the believer. Some of the arguments on this forum illustrate believers exist here. I'm not sure we'll ever return to mass acceptance that one's truth is subjective in nature. My take is we are in ascendent phase of confusing ideas with belief, we'll be seeing ever more overt and ugly attacks on each other into future.