Disclaimer: This answer is not at all guaranteed, but it isn't really technically wrong either.
The voting system is the main system that the Stack Exchange Q&A network depends on, it helps to show what is good and of quality and what is bad and of low-quality. Downvotes are as important as upvotes and some users may not acknowledge that idea.
When should you downvote? Simply put it if you find the question lacks effort, is very badly written, poorly researched, or has a very low quality you can down vote it and flag it if needed. Some questions may look bad, but maybe easily fixed by editing if you see those you can edit the question to fix the problem instead of downvoting.
If you casted a downvote on a post for a problem I listed above and it was fixed you can retract your downvote.
You don't really need to leave a comment when you downvote, but it is a good thing to help notify the user of the problem.
For more information on voting see: When should I vote?