I've probably down voted someone less than 20 times on HN, but when I do I like to leave a comment as to why. Its been the case where I've been down voted and wondered why.
His comment isn't a troll comment and looking at his past comments he seems to be reasonable, intelligent, and otherwise a productive member of HN. I wanted him to know why I downvoted him, and he can consider changing the post to offer more insight in which case he would attract more upvotes and would generally make HN a better place.
What makes my comment more productive is that I articulated why I down voted which is that it lacked any insight into the process and didn't contribute to the conversation.