Sometimes it happened to me to see questions/answers downvoted without a comment about it.
I think it is more useful for the user who gets a downvote to understand why he/she got it instead of just loosing reputation points. In this way that user can understand what was wrong with the question/asnwer he/she posted and avoid to make the same error again, whereas at the time being the downvote is useful only for the readers to understand which quetions/answers worth the reading.
So my question is: is it possible to add a rule about this? I mean, make the comment mandatory in case of downvote to explain what was wrong with it.