Reminder to those down voting this comment. Down votes are for people not contributing to the discussion in a meaningful way. They are not for expressing disagreement. Use the reply button if you disagree with the parent comment.
Because upvote and downvote seem like they would be opposites, it's not surprising that people might focus on the negative of "I agree with this comment" instead of the opposite of "this comment is helpful/insightful". The former is easier to measure than the latter.
That's not a rule on HN (unlike e.g. reddit), neither officially nor by user consensus.