It takes a huge level of maturity to know when you understand something. You have to take yourself away from how pleasant the interaction was, and ask yourself questions that are on the limit of what you think you can answer. That whole not-too-easy-or-hard balance is really difficult to nail down, especially if you have a grade depending on it. It's also hard when you have nothing but your own satisfaction depending on it, eg after you've graduated and are just reading for interest.

The entertainment aspect is hard to get away from. It's like when you watch a good documentary, you're in awe of whatever field it's about. But have you really learned much? Hard to say.