According to my idea, downvoting without a comment is not helpful neither for the user nor the community.
Do you see any constructive feedback by just downvoting?
Just downvoting would not let the user to improve. What do you think if we make a comment mandatory if we downvote for a question, but not for an answer?
down voting without a comment is not helpful to both the user and the community
That might be a little true. The down-vote means it was not useful or a low-quality post. But why the voter thought so isn't said.
The reason why you can down-vote without commenting is that voting is anonymous. Do you think it is constructive to up-vote without a comment? Well, it says the voter found the post useful, but not why. Despite that I have never heard people complaining about this...
Usually you will find out the reason quite soon, since the question gets closed or answers within seconds by 5 people. That is a clear indication you should have done a better job researching.
Making a comment mandatory won't help since people would just write because. Also, it takes away an important part of voting: anonymity.
