Downvoting to disagree is my knee-jerk reaction, too. But in most cases it's not as helpful as leaving a comment.

Any time I leave a comment, there is usually at least some thought and effort behind it, and a downvote without explanation doesn't teach me anything, or persuade me to change my mind.

If I don't know the reason for the downvote, my frequent assumption is "this person has poor reading comprehension, or isn't very bright, and just didn't understand what I'm saying. People often dislike things they don't understand. I guess it's true what they say: judge a man by the people who disagree with him."

That's completely unfounded, sure, but lacking any additional information, it's as valid as any other conclusion about why I got downvoted. Maybe I've made some obvious error, but how would I know? In general, getting downvoted tends to make me less respectful of other people on HN, and more confident in my own (flawed) reasoning, compared to getting a thoughtful disagreement.