How does a Fighter develop Heart and Courage?

Any fighter that wants to be successful has a slight bit of heart and courage. That drive to keep fighting even if you're taking the worst beating and pain. But sometimes that drive isn't enough and that pain and suffering triumphs over that drive. Some fighters are just naturally gifted with that courage and drive. Some fighters don't get that chance. So how can a fighter that lacks heart strengthen that desire to keep fighting no matter what he's going through in the fight? Does a fighter just develop that drive over time as he begins to understand the philosophy of fighting or a conflict? having that mindset of a fighter is something very difficult to understand and master. How does one develop heart and courage?