That would have the unfortunate side effect of increasing negativity, as people are pushed to actually voice it instead of giving a harmless downvote. And when you consider the extra effort required, the downvote itself essentially becomes meaningless, turning this into a mechanism of "if you don't like it, make sure to let everyone know why, or shut up" which I don't think is very helpful.
That said, maybe something like adding an extra step where you choose a reason for the downvote from a list like "spammy, offensive, rude/toxic, annoying, misleading/innacurate" etc. or write your own, and those could be visible to moderators (anonymously) as a sort of micro-report. Would dissuade people from mindlessly downvoting and help improve moderation in the process.