Upvoting things you agree with is fine and a main use of the function. Why then is downvoting things you disagree with wrong? Because the purpose of voting is visibility, you upvote things you want people to see, like arguments you agree with, and downvote those you don't think people should see.

Now if you believe in having an open discussion you don't want to suppress posts just because you disagree with them. Disagreeing is fine, so downvoting is reserved for posts that detract from the conversation.

[–] Swedneck@discuss.tchncs.de 3 points 2 weeks ago

you upvote a comment saying they love cats
you ignore a comment saying they don't like cats
you downvote a comment saying they hate cats because cats are evil
and you report a comment saying they eat cats