Of course there is no way to enforce commenting on a downvote, but shouldn't a downvote always be accompanied by a comment why you think a post is wrong, or how the post can be improved to revert the downvote. Downvoting is useful when a post is offtopic (but then the question should either be migrated or closed), or blatantly wrong, but in that case there is cause for an explanation for the downvote. Is this something we should expect from our users, or is it just my hurt feelings after an anonymous downvote today? ;-)
As Mew's comment indicated this is a well-known issue.
A search of Meta Stack Exchange found this question (with 494 upvotes!) marked as status complete. (I could not think of a way to filter out questions with requests to support downvoting comments from that search.)
The solution implemented is to provide the message "please consider adding a comment if you think this post can be improved" to users with less than 2k reputation when they downvote.
People down vote for a variety of reasons, some good, others less so.
There some downvotes for "good," or at least reasonably objective reasons: the post was factually wrong or misleading. Or maybe it was "legally accurate but not volunteering information" (unhelpful). These are the people that are most likely to leave a comment to help you improve.
There are others that vote out of pique, perhaps because of a dislike for a particular phrase, expression, or concept you used, without considering the value of the post as a whole. This kind of downvoting is regrettable, but is tolerated on the site as a "necessary evil."
Then there are people that use what I call "comparative" downvoting. Looking at your answer, it is more succinct, and arguably less complete than a more highly voted answer. It's not a bad answer, but some people might say that it is "not as good" as the other one --and downvote on that basis. Unfortunately, it's not enough for them to upvote the other answer and leave yours alone, but that, unfortunately, is the breaks.
I seldom downvote, and when I do, there's something I consider seriously wrong. I will typically leave a comment when I do, and sometimes, I will leave a critical comment for moderate weakness that IMHO "falls short" of needing a downvote.
Yes, I think we should try to make it the norm to leave a comment when downvoting. It isn't enforcable, of course.
Off the top of my head I can't think of any reason that one would downvote without explaining why... and for those not used to Stackexchange (or even those who are), there are few things more demoralising and more likely to put somebody off contributing good questions in the future than to spend time crafting a question and then see it downvoted (or worse, closed!) without any explanation.