I am new to the forum and I got a thumbs down (minus) to one of my questions. It is a good idea if the person can give the reason for the thumbs down.
Yes it generally is a good idea: it can be helpful if they can and do leave a comment.
Voting is anonymous and it's impossible to require a user to do to that: we can only ask them to.
I edited your question to include the additional explanation which you posted in comments. I think the question is clearer now.