When customers ask for refunds, they have an opportunity to leave a comment. You can see any comments they have left here: https://leanpub.com/YOURBOOK/refunds. You can find this page in the book tools for your book if you go to where you work on your book in Leanpub, and then click on "Selling & Royalties" in the menu on the left, and then select "Sales, Refunds & Royalties" and click on the blue "refunds" link near the top.
Every author has their own approach and preferences, but our general advice is not to worry about refunds unless you are getting quite a few and there is a pattern, in which case you will almost always get enough comments with the refunds to point you in the right direction.
Some customers know they can get a refund and so buy books just to check them out. This is the equivalent of someone taking a book of the shelf in a physical bookstore, leafing through it and putting it back on the shelf.
We believe this increases sales overall, but it does mean you'll get the occasional refund. Sometimes customers just make a mistake about what kind of book they're buying, and the refund is not an expression of a negative opinion of your work.