I've been surprised sometimes how an entire thread will form under a comment of mine, and in the end there will be 2 or 3 sub-comments voted at 20 while mine stays at 1. Shouldn't I, in those cases, receive some votes for starting such a discussion?
If your comment was actually well written, or useful, or whatever people decide deserves upvotes, then you'll get them for it. If it wasn't a good comment but it happened to start a discussion, credit goes to people having an interesting discussion.