I agree with your comment about basic courtesy. However, I disagree with your idea of making obligatory. If people choose to downvote without comment, attempting to force them to comment will be futile. You can't legislate good manners.

Consider - how do you enforce such a thing? You require that the person enter some text. OK, so (s)he enters some random keystrokes, and you're no better off than before, except perhaps knowing who it was who dissed you. And then what? You downvote anything they write? That could lead to a whole lot of feuding over nothing. It just opens up a whole can of worms, without attaining the desired result.

Still, I agree that it's not particularly helpful to downvote someone without explaining what it was that you found objectionable. That doesn't give you the opportunity to correct your behavior.