No. You cannot trace upvote/downvotes since it happens anonymously. Even our moderators cannot see this information, I believe.
I think this is intentionally put in that way in order to avoid further problems. If we came to know who are the people who downvoted our question or answers, then there is a natural tendancy of "revenge". Similary if we came to know who upvote our question/answers, a natural tendancy of supporting them by upvote or promote their activities will come into place. It is to avoid this, I believe, Stack Exchange team put this information as anonymous.
I believe this info is available only for stack exachange development team.
It is not required to put a comment after upvote or downvote a question or an answer. But it is a good practice to mention what went wrong in case of downvote. But we cannot force users. Only thing you can do in this case is, ask for clarification in case of downvote.
If they give the explanation, whether that explanation is correct reason for down vote or not, the author who answered or
questioned can understand the need and they can edit as per need.
Ya, to improve an answer, a comment may help. But there are lot of users who are here only for gain points. They will copy paste an answer from somewhere and even if we comment, they don't know how to improve that answer. Another situation is, there are multiple good answers available in a thread and someone provides an answer that involves some rewrites, but still works. People will downvote such answers since it is not the "right way" to do that (even though it may resolve the problem).
So this platform is intended to use benefit others and us, not in terms of points, but in terms of knowledge or a service. If you are around stack exchange site for a long time, you will see lot of such false behavior from lot of users and you fed up with those. So there will be a natural tendency of "silent moderation" where you will upvote/downvote or review in strict way but without make any comments.
So if you want to do some really good stuff and want to improve your answer, then always be open to upvotes and downvotes. If you get a down vote without a comment, then that means someone thinks your answer is not upto the mark. So try to learn that stuff in more detail and improve your knowledge.
But still, as you are pointing out, a comment would be great if we put a downvote. But we cannot enforce it and we should not be agitated because of it, rather become more vigilant. :-)