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. 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.
