Whenever I get downvotes, I stop to consider whether or not my comment has added to the conversation. If I think it has, and I can't figure out why I got downvoted, I just move on.

Of course now I re-read all my comments right after I post them, so I tend to delete quite a few before voting happens either way.

I would guess for the notion of creating another HN account to sockpuppet/additionally up-rate someone.