I think there are many people around here who downvote questions and answers based on there mental state, to relieve their frustrations or because of personal hatred or something.
For example, I have received 4 consecutive downvotes to my popular questions and answers a few minutes ago and all within 2-3 minutes. Which makes it clear that someone is deliberately downvoting my popular posts by visiting my profile.
If those downvotes are genuine, I strongly welcomes it and it will be appreciated if comments about the reason of downvote will also be posted so that I can improve the posts or improve myself when I post a question or answer next time.
So in my opinion, commenting reasons should be made compulsory, so that then only a community member can understand their mistakes and improve themselves. IMO a huge number of downvotes without any comments (reason), will never help the community users or the community to get better. They will only get frustrated and lose their confidence by this kind of action. But if the reason is also provided. That will let them understand their mistakes. Otherwise, they will think this place is not for them and people are deliberately downvoting them and eventually they will stop contributing to the community.
A related meta question: Can we comment the reason for downvotes? For a few days at least?
But my suggestion is to make commenting the reason compulsory for downvoting.
Note: Feel free to edit and improve.