I've noticed a lot of questions getting downvoted with no comment or clarification on how they can be improved or a reason for the downvote. Its not required, but it would be nice if folks would leave a little note in the comments telling the OP why they have downvoted.
This topic has been hashed over many times on many SE sites. The short of it is that commenting on your own down-vote essentially removes your anonymity from that vote. Anonymity is an individual right that is central to the integrity of any public voting system, and not everyone is comfortable with giving up that right.
I personally am very much for adding commentary to go with a down-vote, or up-voting existing commentary which applies. Otherwise, the down-vote is not really serving any constructive purpose.
Again though, not everyone is as comfortable giving up their anonymity in the voting process. We'll just have to accept that and respect their rights.
I believe that if you are downvoting something, there should at least be an explanation as to why, unless someone already explained for you.
Downvotes, while they may be anonymous, I suspect are as such because listing everyone who voted on everything would be massive clutter. It could be done, but there are practical reasons not to (and I don't intend on speaking for the admins here, I'm just speculating).
I've found a few of my answers being downvoted. I asked about one of them in chat today and someone (who had not seen my answer to that question before) gave his opinion as to why it might have been (or why he did). I agreed and edited accordingly. However, just downvoting with no comment as to why the answer isn't good isn't helping anyone.
Given the nature of the topics discussed, and how some of them are historically divisional, I can't help but wonder if people are simply downvoting a post that they don't agree with, even if it's a valid answer from a particular point of view, and isn't overstating itself. If that's not the case, then commenting would only serve to make the answers better quality.
Why seek anonymity on your downvote? If you are going to voice your own opinion in a vote, then stand by it and defend it. Perhaps you may change someone's mind or clarify a point by doing so!