I disagree with the premise that every time you downvote, you must always leave a comment. Personally, I used to do that all the time. However, I experienced all sorts of negative reactions: people got upset, they revenge-downvoted my other answers, they flagged my comments as offensive (even when they were neutral and factual). After repeated exposure and getting burnt many times, I've learned my lesson. I know that many others have come to a similar conclusion as well.
While I appreciate others who take the time to comment, and I often do try to comment in a helpful and constructive way, I cannot agree with the suggestion that it is always rude to downvote without commenting. I think the situation is more nuanced, and there are sometimes valid reasons to downvote without commenting. (And don't forget, both upvoting and downvoting are part of the StackExchange model.) While we don't want to discourage all new questions, frankly, I suspect we do want to discourage questions that are off-topic or poorly-explained.
Personally, I think it is the poster's job to understand the culture of the site, read through the help center, read some other questions, and then post a well-posed question that makes it self-evident how the question is on-topic (e.g., in what sense it is a research-level question). While I'm often happy to try to help even new posters who don't do that, I don't agree that it is our moral obligation to do so for every single new poster.
How can the poster know to improve the question? They can read the help center. They can look at other questions that have been posted here. Or, folks (like you) who see something positive in the question can leave a comment suggesting how the question could be improved, or even edit it to improve it.
If you want to help new posters and ensure they aren't discouraged, the best things we can do are:
Leave a comment explaining (constructively and helpfully) explaining the expectations of the site and how the poster can better meet those expectations.
Edit the question to make it a better fit for the site, and to make it clearer why it is a good question that is on-topic.
As a bonus, you might also notice how these are things that you can do, that don't require you to convince anyone else to change their ways. That's a positive thing: that means these are actually implementable. (In contrast, trying to convince everyone to change their ways is not.)
I realize this may be an unpopular message, and I expect to be downvoted (and that's fine). But since you asked, I'm trying to share another perspective.