Is there a Brain Mechanism for Reluctance of Studying?

As in everything in psychology and the studies of behaviour and the brain, there are many interpretations for the extremely complicated process of learning. If you want to check a basic (But detailed) book on most of the early understandings on the subject of Learning, I recommend anything written by Michael Domjan; specially "The Principles of Learning and Behavior".

Also, the studies showed in "Cognitive Psychology" by Robert Stenberg (Mainly memory and perception) could shed some light. If these books are not enough, you could always look for more specifics in some books in neural science, like: "Principles of Neural Science", written by Eric Kandel (Chapter 9 covers mostly learning, but I still recommend Chapters 1, 3, 4, 7 and 8)

If you want a more casual answer, learning is something we don't wanna do sometimes because it is very frustrating, exhausting and hard. Even more, if you're trying to study a very difficult subject, where you start to acknowledge your own cognitive limitations and falling victim to our impatience.

Hope this helps.