Fun is just another word for learning.
Fun from games arises out of mastery. It arises out of comprehension. It is the act of solving puzzles that makes games fun. In other words, with games, learning is the drug.
There’s a good case to be made that having fun is a key evolutionary advantage right next to opposable thumbs in terms of importance. Without that little chemical twist in our brains that makes us enjoy learning new things, we might be more like the sharks and ants of the world.
The single-player game is a strange mutant monster which has only existed for 21 years and is about to go away because it is unnatural and abnormal.
In other words, with games, learning is the drug.
When do you do things for the love of it? Always. If you’re not feeling that, go do something else. Life is too short.