I agree with what Wolf says above. Many forums are decidedly more hostile than here, although that doesn't mean that we're ideal by any means.
Hostility in politics has a long history, going back before the internet. The gangway separating the two sides of the House of Commons is "two swords length" wide. That isn't an accidental choice of words.
Nevertheless, we do find ourselves in a new social environment, posting anonymously as we do. The usual social restraints associated with the presence or even the voice of others in a discussion aren't there. From the human animal POV, I'm the only one who exists in that sense as I type. I can't read your body language, can't hear
how you say things. Of course I know intellectually you exist, but when I type am I addressing the OP, or everyone, or just typing to say what I want and not caring at all? Am I doing this to put forth intellectual arguments or emotional ones? How do we effectively differentiate between the two? Am I serious or am I just trying to ratchet up tensions because I enjoy trolling? Indeed, am I what I appear to be? I could get a kick out of making posts for the sheer delight of annoying others.
There's a whole lot of uncertainty when someone posts. Things are easily misunderstood, the topics in politics tend to be things which naturally invoke strong responses.
We are dealing with a medium that encourages the worst in human behavior coupled with charged topics combined with no real time feedback that we as social creatures have depended on since the beginning of our species. Add to that we have no idea who our real audience is and we have what you see.
The internet is wonderful for the inane, but serious topics? It's highly flawed.
Then we have the arch nemesis of "facts", confirmation bias. We tend to automatically seek viewpoints which reinforce our POVs without even realizing it. We then find what we want, which is self-validation. Once we are certain we are right (after all, there are many "factual" sites which prove we are correct) that sets opinion in stone, and leaves us unyielding.
It takes considerable effort to realize that oneself is not the center of the universe, that other ways of thinking may be equally valid, or in fact correct while we are not.
We tend to look at things and parrot them. "This policy is good for America" without much understanding of the topic, which I find personally irksome.
So this is P&N, and why it is as you see it. At least that's how I view things. I could always be wrong of course.