> Quoted from [Gilles](http://meta.askubuntu.com/users/1059/gilles)'s answer on [Down voting consequences](http://meta.askubuntu.com/questions/2008/down-voting-consequences) ahead of this question being marked as a dupe

When you downvote, a pop-up appears suggesting that you leave a comment. Mandating a comment has already been proposed many times on the [main Stack Exchange meta site](https://meta.stackexchange.com/). It was shot down because

 * votes are anonymous, so you should not have to reveal your identity when you downvote;
 * often you're not the first to find the flaw in the answer, so you would upvote an existing comment rather than post your own;
 * sometimes “you're way out there, read the other answers” is the only thing you need to say, and that doesn't really warrant a comment;
 * you can't realistically prevent someone from posting a comment then immediately deleting it.

In short, downvoting carries a symbolic penalty (1/10 of an answer upvote); you are encouraged, but in no way obligated to comment.