Whenever you begin learning a new subject such as maths, science, programming, a new language, or the piano, your enthusiasm level soars. Basic concepts are simple and easy to grasp, everything is fresh and new = this is fun!
Then comes the trough where simple things, just aren’t so simple anymore. Concepts become more complex and harder to understand, everything is old hat = this is not fun anymore.
With work and perseverance, you eventually master the more advanced material and pass the inflection point, and gain confidence. Advanced concepts become easy, you get to apply the knowledge in new and interesting ways of your own choosing = this is way more fun than it was before!