This has been mentioned by users in various questions, and having personally seen it to my question(s), I wonder whether it might be a good idea to make posting a comment explaining a downvote a requirement to down vote a question or answer. This makes it easier to understand the reasoning behind the vote.
That said, by the same logic, it ought to be needed for a up vote too. However, an upvote is understood to mean good quality of question by default. So, my question is specific to downvotes.