Possible Duplicate:
Should we force adding a comment with every down-vote?
Why not let the site force the user to enter a comment (of min. 20 chars. or so) when demoting an answer?
Possible Duplicate:
Should we force adding a comment with every down-vote?
Why not let the site force the user to enter a comment (of min. 20 chars. or so) when demoting an answer?
Anonymous voting is a key part of how Stack Exchange sites work. If we assume for a moment that you are not suggesting revealing the names along with the comments, forcing people to explain their downvotes still would not get you any usable information. Once you make those comments anonymous, you also take away any sort of accountability for content, so in most cases you'll end up with throwaway or gibberish comments like mine. (Although is intended specifically to illustrate the point.)
This has been brought up many a time on Meta Stack Overflow. The request to force anonymous commenting was declined. The request to encourage explanations was implemented via a pop-up that every user sees when they downvote a question or an answer.