People downvote with no comments, gives no idea what is wrong with an answer or question. If revealing the identity of downvoter is the concern, please at least allow a comment for downvote anonymously.
Requiring people to do things will. Never. Work. You cannot require people to leave comments when they downvote:
This creates spam when someone gives a really bad answer. Do we need 20 comments saying "Your answer sucks" or would one comment with 20 upvotes serve the same purpose (with a lot less flooding of the comment thread)?
There's no way to prevent people from leaving useless junk comments (like "asfdsgjhadsgasfsd" or "+1 great answer") to avoid having to explain their downvotes.
I always try to down-vote with a comment, which I think helps the community. However enforcing someone to leave a comment (even if 'anonymous') may not yield meaningful message. It probably discourages down-voting.
A down-voter is getting a hit of -1, that 'sacrifice' should mean that they are not likely to be down-voting for fun or abusing.
The current situation of using a reminder is sufficient (at least that's how I feel).