No, comments should not be required.

First and foremost, comments can't be required. There's absolutely no way to enforce it, without rewriting the code.

While I believe comments should be encouraged, they can't be made mandatory.

Nor do I really believe they should be.

Revenge downvoting and arguments in comments are very real possibilities for someone posting even the most constructively-phrased comment explaining why a negative action (downvote or VTC) was made. Some people don't want to be involved in conflict like that, and that's their right. They shouldn't be stripped of one of the fundamental forms of participation on this site just because they can't trust other people to respond positively to criticism.