One of the leading theories to categorize elements of games is the self-determination theory. The theory claims that we are all to a certain extent self-motivated to undertake actions for our own well-being. Learning is no different. Knowledge can give you an advantage in life.
You can either be self-motivated to learn something because of the intrinsic reward of fulfilment an action brings or extrinsically motivated because of rewards you get from an action. Self-motivated learning is stated to be the most healthy way, but the tricky part is in staying motivated.
More on that another time.