In online communities where so many are anonymous or psuedo-anonymous, it becomes easier and more mentally healthy to downvote an opposing position than to reply with a well reasoned response only to find out your dealing with a bot or someone who insists the sky is green.
I'm not sure how we fix this, though the signal to noise ratio is higher here than many other sites, so I keep coming back.
PG said it was OK once, because upvoting for agreement is also OK, and now it's permanently baked into the culture, despite the obviously incorrect assumption that merely because the actions are symmetrical, their effects are also symmetrical.
It's also funny that people have been saying HN has been going downhill or "turning into Reddit" since the beginning. It's common enough that it is (or used to be) specifically barred in the guidelines.
I think there's a tendency for many people to consider everyone who came to a culture before them to be authentic, and everyone who came after them to be the ones destroying it. The Eternal September effect is real, but it also panders to nostalgia and a sense of entitlement that says things were better when we and our culture were more relevant.
I personally try not to downvote a post I disagree with, if I think it's a valid, reasonable contribution to the discussion.
The system I've proposed is: you can downvote a post if you've upvoted a reply to it, or if you have replied.
If somebody is breaking the rules in clear bad faith, that's what flagging is for.
That was never the intention behind downvoting privileges if I remember right. Down voting exists to bury flippant, inappropriate or insulting comments.
We really cramp quality discourse when we automatically hit down just because we disagree.