Actually it is neither forbidden nor uncommon to see a comment by the downvoter explaining their reasoning for the downvote. But complaining about being downvoted, or inviting discussion about it, is frowned upon.
I believe however that this is one of the fundamental flaws of Reddit-style websites. On an imageboard you are obliged to make your disagreement in words, constructive or otherwise. Here, you can passive aggressively click the downvote button and leave the post in limbo.
(I am not at all talking about the present case, let it be clear :-) )