Not really hurt, but it does help me understand how much they're interested (or not) in what I'm teaching them.

Yes, sort of ... it hurts that they are hurting themselves by neglecting what is needed. As a self defense and health science instructor, I can tell when there is excessive practice, adequate practice, and less than adequate practice. I try to emphasize the difference between vague understanding and complete mastery and that the difference could be life or death ... it hurts me a bit to see that "this" student will not be the next grand master training students of his own and helping to both gather and spread both health and wisdom.

I like to ask my students if they have learned "today's" lesson well enough to be teaching it tomorrow ...