I recently tried to un-upvote a comment, and received a message that if I did so, I would not be able to upvote it again:
Are you sure you want to undo your upvote on this comment? You will not be able to upvote it again.
Unlike this question, I completely understand what the message is saying. However, why am I not allowed to upvote it again? What's the reasoning behind this decision?