If you think that a post is worth to being up-voted, you can up-vote it. There isn't much the community can do about the down-votes, as the only user who can remove the down-vote is the user who casted it, if it's done in the 5 minutes period, or if the post has been edited.
There isn't a way to leave a comment for down-voters that is automatically notified to them. Down-votes are free to leave a comment, or not; when they don't leave a comment, it could be because there is already a comment with which they agree, or because they don't have anymore to see, apart what reported in the tooltip for the down-vote button.
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Anonymous down-voting is part of the system; if the down-voters should always add a comment, there would not be anymore anonymous down-votes. I don't think it would be productive to force the down-voters to add a comment. What would be the comment if the down-voter agrees with a comment already left from another user? We surely don't want a set of "I agree with [username]" comments.