The upvote/downvote should serve the utility of promoting/suppressing relevant/irrelevant content to the discussion. It's a "vote what's relevant", not "vote what you agree on" button.
Also, if you disagree with someone you can just move on, you don't have to answer.
From HN's guidelines:
> Users should vote and comment when they run across something they personally find interesting—not for promotion.
From Reddit's Reddiquette:
> If you think something contributes to conversation, upvote it. If you think it doesn't contribute to the community it's posted in or is off-topic in a particular community, downvote it.