A downvote isn't the opposite of an upvote. An upvote can function somewhat like a teacher giving a student a gold star, but for the most part it primarily highlights good content (not necessarily that you agree with the content), above average content. A downvote, however, also functions as a punishment, a demerit, a warning. A message that "We don't want your kind around here", with real consequences (getting banned, etc). The problem is that it lumps inane content (or "graffiti" content) with posts that are an unpopular opinion. I've often seen downvoted comments that still end up generating a lot of interesting discussion, and I'm glad that those comments were made.

But since a downvote is a punishment, often times people will second guess if they should post something interesting just in case it is taken the wrong way by someone. That's happened enough to me that now I very seldom comment on any thread that has any type of controversy in it, just because I know that someone will get upset no matter what. (Maybe a fix for this is to make it free to give upvotes, but downvotes can only be given if you've purchased them with some of your own karma points).

An upvote should be a sign that the comment-er did a good to great job of explaining a position that may or may not align with your confirmation bias and other thoughts, not a simple "I agree" signal.

Because any online community where that attitude prevails degenerates into an echo-chamber.

Voting is meant to promote quality contributions and increase the overall quality of the site. A site where dissent is pushed out of site would be very dull.