This is a group of philosophy. Made by and for philosophers. Philosophy means a taste, a fondness for knowledge; a will to go out and seek knowledge.
So, if someone downvotes the question or answer of someone else, he/she should: 1) justify why he/she did so, or 2) upvote another justification he/she agrees.
If you think the other is wrong, tell us so! Explain! Help him/her to raise consciousness of the world. Engage in discussion, maybe YOU are the wrong one.
So my question is: should commenting (or at least upvoting another comment) be mandatory if someone downvotes at philosophy.stackexchange.com?
PS: Read somewhere else that "We will never be able to require explanations for downvotes"
"Never" is just too much time. Of course it's possible, it's just a matter of collective will.