Quoted from Gilles's answer on 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. 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.

Quoted from Gilles's answer on 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. 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.