Original Meta Stack Overflow answer
Forcing downvotes to be accompanied by comments sounds like a good idea at first, and many here would like to see new users get all the info they need to ask questions that are a better fit! Contrary to popular opinion, most users here are nice and want to help, and don't enjoy "shooting down" newbies' questions with downvotes to make them feel bad and unwelcome.
However, Stack Exchange sites are big cities. Big cities need different rules to survive than small villages - and tend to feel more anonymous and harsh as a result.
Downvotes are important for the health of a site, and mandating comments for them would massively impede the way Stack Exchange currently works — to the point of potentially destroying it. It's just not feasible, for a number of very good reasons. That's why, although this gets suggested frequently (on average, a few times per week overall both here and on all per-site metas), it is declined and often downvoted* by meta users.
Here's an overview of the primary arguments for why it's a bad idea:
Downvotes are, first and foremost, a content rating system. Rather than being a way of communicating with the author, they are a way of communicating to future readers that a question or answer is not useful, doesn't show much research effort, or is not a good fit for the site. If someone wants to leave a comment to communicate with the author, they can always do so, independent of the voting system.
In the vast majority of cases, nothing needs to be clarified. The tooltip on the downvote arrow already explains what a downvote means, and it is specific for questions and answers. In most cases, the "comment" in the tooltip already adequately explains the logic behind the downvote, so an additional comment would just be wasted effort and noise.
Any requirement could be trivially circumvented by entering gibberish or something unconstructive like "this is bad". Detecting and stopping those who enter such stuff through moderation/administrative action is simply not feasible on a site with millions of users.
It may not feel that way to you at the moment, but downvoters are doing the site a service, and making voting more difficult would impede the site's most important quality-control tool. Voting is ad-hoc and frictionless by design! Voting separates good content from bad, and makes the good content more visible. This is essential for the platform to work, even if it sometimes feels mean. If a vote is in error — which can always happen — the expectation is that the "swarm intelligence" of future viewers will eventually correct the problem. A single vote is nothing, really. What matters is the sum of all votes, which is why we only display the aggregate score.
Scale. Stack Exchange sites get thousands of questions every day overall. Many of them are of poor quality, or just not a good fit for the site they are asked on. It is beyond human capability to respond to each one of those bad or misplaced questions with custom-tailored advice. It would drain too much time and energy from the unpaid volunteers who answer questions and help users.
If downvoting is made more difficult, then upvoting would need to be made correspondingly more difficult. The system uses downvotes and upvotes to filter out the "good" content from the "bad." If consequence-free downvoting is a problem, then, logically, consequence-free upvoting is, too, because it potentially marks low-quality content as "good".
Documentation on how to ask a good question is made easily available for those willing to read it. Stack Exchange's rules are special and arcane, but it's not like there hasn't been a lot written on the topic; in addition to the help pages, there are also comprehensive FAQ questions written by Meta community members, with the faq tag. Similarly, we provide extensive guidance on how to answer questions.
Leaving a comment accompanying a downvote can lead to negative consequences, like revenge downvoting and even off-site harassment. Many experienced users will tell you that they used to leave helpful comments along with their downvotes, but have stopped doing so because of the unpleasant blowback they received from some unreasonable users. Even for those users who remain rational, commenting about votes almost inevitably leads to extended, off-topic discussions, which we strive to avoid.
Stack Exchange, Inc. is actually doing a hell of a lot to make the place feel more friendly. The past few years have seen tons of discussions, initiatives, UI changes, help center updates and renovations, experiments like mentoring, and more—all aimed at making starting out on Stack Exchange a more pleasant experience without compromising on quality. (Actually, a lot of veteran users feel that site's owners are putting too much emphasis on making the site feel nice, for the sake of traffic — which translates into money — over quality; regardless of whether they're right or not, it is not accurate to say nothing is being done. It's just a really tough problem.)
We can't accommodate everyone. There will always be more question-askers out there than there are competent answerers. You can't overburden the latter by allowing a huge quantity of bad or badly-fitting questions into the system—you'd destroy the entire system, and hence prevent any questions from getting answered. Not getting to ask your question on Stack Exchange isn't a death sentence; many veterans have questions every day that they don't ask on Stack Exchange because they know they wouldn't be a good fit under the current model. The resources those veterans turn to to solve their problems are usually open to everyone on the Internet - they just take time, effort, and sometimes periods of frustration to understand. There's also other, more mentoring-oriented resources to turn to.
* On meta sites, downvoting is used to indicate disagreement with feature requests. But it's the same thing: the votes are a way of communicating to future readers (including the Stack Exchange team) that a certain feature request is overall disagreed.
