I never see anyone complaining when they receive an up-vote without an explicit explanation...
As Iain is fond of saying: "No one who needs to, reads anything least of all documentation."
Many first time posters skip actually reading the the site introduction and the how to ask pages and that is unlikely to change. In that light a mandatory/automatic explanation for a down vote will be mostly useless.
At best we remediate that for users who care, which is why I make use of the StackApps AutoReviewComments extension and insert the following comment whenever I see an user actively questioning the reason for a down vote:
By philosophy and design votes are anonymous and neither voting up nor voting down requires any mandatory explanation. The tooltip that appears when your mouse pointer hoovers over the down button states: "this question does not show any research effort; it is unclear or not useful". Also questions can attract a down vote when not well written, not quite on-topic or missing details.
That minority might improve their current question or will ask better questions in the future.
