I am not sure if I am imagining things, but some people might be downvoting questions illegitimately, so in order to prevent them from doing that can we force them to comment and tell people what's wrong before giving them the right to downvote, because I don't think users should discourage people from asking questions, because it's the lifeblood of the website.
Down-voting is a fundamental part of the SE ecosystem
Down-votes are a core part of the site. Requiring comments for downvotes is bad for a number of reasons. This question has been asked on every meta on SE at some point. Here is the official main meta question for your information.
To summarise the top answer:
- Votes are a content rating system, downvotes are a required part of that
- Comments don't always add anything the downvote did not
- Comments can lead to negative reactions
As to why this question is receiving downvotes, see the tool-tip for downvotes:
This question shows no research effort, it is unclear or not useful
A quick google search of this question would have brought up the linked question and a number of others from various other SE sites.
Yet another issue: you do not actually give people a chance to comment. Twice I downvoted your questions, and was in the process of leaving a comment and explaining why, when you deleted the question.
If you believe that "some people might be downvoting questions illegitimately", why would you delete the question? Wait, and some other users, who see your question as a good one, would upvote it. For example, my own Why are there no Jews in the Star Trek universe? on neighbouring Scifi.SE has 13 downvotes. And 58 upvotes.
For cases that are really illegitimate, as in serial downvoters, it is considered a form of harassment. It gets caught by the system and reversed. Which again means, you shouldn't be deleting a question just because it got a downvote.
As for "not discouraging people from asking questions", a downvote isn't meant to discourage. I a system with ratings, if you have upvotes, you've got to have downvotes. As @linksassin says, they're the lifeblood of the system. Once in a while, someone would consider your question unclear, or not useful. Maybe they think your underlying premise is wrong. Once in a while someone will disagree with an answer you give. Are you discouraged every time someone doesn't see things the same way you do?
In short: this won't help. You can lead a horse to water, but you can't make it tell you how the water tastes.
I understand where you're coming from. It's frustrating when you get unexplained downvotes. I had it happen to me on Arqade.SE last week and had to restrain myself from demanding an explanation. That's why I always try and explain my downvotes, close-votes, and delete votes. I even try and leave comments on posts I haven't downvoted, to the effect of "I didn't downvote this, but the downvotes may be because..."
Your proposed solution - forcing people to comment before downvoting - has been suggested countless times. Unfortunately, the answer will be the same every time: no. Not only would this fail to solve the problem, it would actually create a whole new problem: comment spam.
As Monica Cellio demonstrated with her first comment, there's no guarantee that downvoters, if forced to leave comments, would actually say anything useful. They may instead just enter a keyboard smash, or abuse, or a copy-paste of a previous comment, or the first 600 characters of the Bee Movie script, or... you get the idea.
So not only would you still be none the wiser about why your post was downvoted, but then the mods would have to come along and clean up all those useless comments (and the argument they would likely cause), creating more work for them. Stack Exchange has been trying to crack down on such useless comments - certain SE sites have had the "add a comment" prompt replaced with "suggest improvements", for example - and there's no way they'd agree to any proposal that would generate more of them.
So unfortunately, this proposal just isn't going to fly. Some people are always going to downvote without providing useful comments, and there's really not much we can do to prevent that. Forcing them to leave useless comments would just make everything worse.