If a user downvotes without leaving a comment the best thing you can do is to either just ignore it and move on (this is what I usually do), or leave a polite comment asking if there is something wrong with your question so you can improve it. I find that leaving a polite comment asking why will usually get a response from someone explaining why they think the question was downvoted.
There are a number of reasons why someone will refrain from down voting. In my experience most users will refrain from commenting when down voting for one of two reasons:
- Nothing constructive to say. If a user doesn't feel like there is anything for them to suggest in order to improve a question they may not see the point in down voting. If a user just going to tell you "I think your question is bad and here's why", and there's nothing you can do to improve that, then that comment will probably just irritate the question asker.
- Avoid a confrontation with the asker. Some users will get really hostile when you down vote a question, and may even insult the down voter. This situation happens more often than you'd think (it happened earlier today in fact), and I can't blame a user for not wanting to deal with that.
The main thing to remember is that a down vote is not a close vote. I down vote questions all the time which I don't think are good questions, but are still perfectly on-topic for the site. So just because your question is getting down voted doesn't mean it's going to get closed.