One reason we have a "100% Happiness Guarantee" two-click refund policy is that it encourages people to buy books to try them out, because they feel that a Leanpub purchase is risk-free. So, sometimes people ask for refunds just because they took a look and realized the book wasn't quite right for them. Our position is that this policy, while it does produce some refunds, actually increases sales overall.
Customers have an opportunity to leave a reason when they get a refund, but they are not required to do so.
We don't contact people when they request returns, partially because we want them to feel like there's no friction.
If you are getting refunds because of an issue with the quality of your book, in our experience, you will pretty quickly hear about it from readers, either on Twitter or (eventually) from a return that contains a comment, or they will send a message using our "Email the Author(s)" feature on the book's landing page (unless you have disabled it).