I know how many questions are posted to this website per second. Some questions are repeated or even not clear. However, I experienced that some questions have already had a down vote and I see that these questions may be are interesting for other people recently or in the future. However, anyone who have eligibility to vote can choose down vote just because this question is not under their interest or maybe not clear for them! Some people also, just flag the question directly even without comment if this question is clear or have other problems asking the author to edit their questions. So, is that right? Is that part of user right to do so? Is there a way that this website can figure out this problem? Is it right to leave this under the personal opinion of the user? I mean I can up vote or down vote as I like!! But I think that shouldn't be happened.
I'm sorry for any new user who has been discouraged in genuine attempts to ask a question. I agree there's a problem with what appears to be an amount of fairly indiscriminate downvoting.
If you're posting the best question you can - and try to act on any feedback you do get - I'd encourage you to avoid deleting your questions (at least if they only get a couple of downvotes). At the same time, when composing a question, pay attention to some of the good questions on site and try to emulate their best features.
To respond to the title question first:
If a question is not of interest to you, you certainly shouldn't downvote it only on that basis. That's not the purpose of downvotes. The purpose of voting is outlined here:
which says in part:
Voting up a question or answer signals [...] that a post is interesting, well-researched, and useful, while voting down a post signals the opposite
So the point of downvoting is to indicate questions that are useless, badly researched or of no interest to the community.
Some people may have difficulty in judging the difference between "not interesting to me" and "not interesting to the community" -- our site has a very disparate collection of topics across a huge variety of application areas, each with their own terminologies and favourite tools. If we each downvote whatever is not of direct interest to us personally, every post -- including the ones we would most cherish -- will be dowvoted to oblivion (quite literally -- posts that get enough downvotes get shown as a pale grey, making them difficult to read).
There are many kinds of questions I don't like (the endless post hoc probability questions, like "my girlfriend's birthday is on the same day as my cousin, what are the odds!!" for example, which seem endlessly popular), so I haven't tried to impose my low opinion of those questions without a clear indication from the community that we should treat them as duplicates.
However, we can't be sure what a person's motivation is for a downvote. Whether someone should downvote is different from whether they can; some people will abuse the downvote and mostly all we can do is ask them to try to follow our community norms (which are mostly to try to encourage better questions via comments, edits etc., whether or not a downvote is applied), in order that newer users can actually learn what the expectations are rather than simply be fried under a torrent of unexplained downvotes.
We have had an increase of downvoting in recent times, so this question represents a broader issue. I'd encourage those people who are downvoting a lot* to please - as a general rule - explain them**, especially to new users. They need guidance more than discouragement
* (a few of you have given more downvotes in a short time than I've awarded in my entire time on site ... approaching 7 years)
** there are exceptions; for example: 1) If it's already clear (from a closure message or another comment) what the problem is, then you don't need to repeat the explanation. 2) If the person you're downvoting regularly abuses people who point out problems, mistakes or the like (even if they turn out to be right!), they lose any expectation of having downvotes explained to them. 3) egregiously bad posts that fail on multiple grounds - ones beyond editing into shape - may merit a flat downvote, but even better is a vote to close (or a flag if you can't vote to close)
Now to address some other issues from the body of the post:
I know how many questions are posted to this website per second.
To Stack Overflow, sure. Here on CV? No; we might see 200 questions on some days, and at peak times a new question on average once every minute or two.
anyone who have eligibility to vote can choose down vote just because this question is not under their interest or maybe not clear for them!
Yes, they have that ability.
Some people also, just flag the question directly even without comment if this
I think flagging is not so much a problem, since it would then be reviewed by a number of high reputation users or by a moderator (depending on the particular flag generated)
Is it right to leave this under the personal opinion of the user?
The idea is to have some way to indicate better content. If users don't have the right to vote at all we may end up with a much worse problem than we currently have; it would certainly result in a very different kind of site. Similarly, a forced public vote encourages revenge voting from a particular subset of users -- generally the same ones I suggested don't deserve a comment above.
We need some way to control the increasingly heavy stream of very poor questions that hit the front page or the quality of the site degrades badly. Votes certainly form part of that management. At the same time we need to be able to teach people how to use the site ... and votes (certainly not in themselves) really don't work for that! They just don't -- a new user who hadn't yet found all the guidance on how to use the site properly - the stuff in the help and on meta and so forth - has no basis to even guess what the problem is.
Let me add a personal anecdote. After I had been using this site actively for quite some time (months of active answering), I received a downvote for what I thought was a reasonable answer. I queried it.
The person who downvoted (no longer active, more's the pity, since they were highly knowledgeable and a great explainer) was kind enough to come back and say what the problem was. The explanation was not exactly gentle - the exasperation came through loud and clear - but it was nevertheless correct; I was (ignorantly) doing the wrong thing, perhaps well past the time when I should have known better. I was very appreciative of the explanation and made sure to say so; and - perhaps more importantly - I tried to act on it. The explanation made all the difference, and it contributed to an improvement in the average value of my answers. A bare downvote would have done nothing but continue to baffle me, possibly for many months to come, while I would have continued ignorantly doing exactly the same thing.
How much harder then, for a user on their first day?
Criticism can be hard to take at times, but constructive, polite criticism should be valued. An explanatory comment can be helpful and constructive, but a bare vote can't.