First and foremost, I completely agree with the quote from Shog9 (which Lilienthal posted).
I enjoy being able to down-vote posts I don't care for without worrying about retaliation. And I really enjoy being able to leave honest comments without worrying that they'll be justifiably interpreted as evidence that I've down-voted. I would not like to see the two systems linked.
The following ramble is basically why I agree with that comment.
In general I see two categories of downvoters: people who think your post is bad/in need of improvement and people who downvote for malicious reasons (strategic downvoting, serial downvoting or just because it's the internet and they don't like people). Since we're not going to get anything helpful out of the second group even if we forced them to comment (which I think was agreed to be a bad plan long ago) let’s just ignore them.
Ok, so we have a group of people who have downvoted your post for some legitimate reason, while it's true that a comment from these people could help improve the post, quite a few people react negatively to criticism.
At this point I think we need to stand back and look what a downvote means. In general, a vote is your indication to the rest of the public that the post is either good or bad. If I see a question with a tonne of upvotes, I’ll go read it. If I see a downvoted post I’ll either pass or read though it to see if it needs a flag. Additionally, votes reward or penalize good and bad posts respectively. In other words, votes help improve our site by guiding visitors using past experience and encourage posters to produce high quality content.
So we have (hopefully) come to an agreement that voting is good and we want to see more of it. As such, we should not be putting the voters in a position which discourages voting. If people want to comment on downvotes, great, if they don't want to, that's totally ok.
I’m also going to add that the top 3 voters on the workplace all have more downvotes than upvotes. They are the people on the front lines battling low quality content and if they even received 5% or people getting angry at them over a downvote, the would have hundreds of people yelling at them.
Finally, downvotes carry next to no penalty so it’s really just a gentle slap on the wrist and a reminder to really think about ways to improve your post.
Ps. This is just my opinion, not a canon answer or anything. I have no idea what I’m talking aboutTM