I'd argue that this has a lot with to do with set point theory.
As humans, our curiosity arouses around novelty. The object will tend to lose its novelty over time, and we'll tend to progressively lose interest in it up to the point where we go back to 'set point'.
Hint: look for mechanisms in your side projects to add novelty to the system so that your interest doesn't plateau.
When we built https://sametab.com as an internal side project, we used to constantly brainstorm new ideas about how this could eventually become. The more ideas we were founding the more the project looked new and appealing at our eyes.
As humans, our curiosity arouses around novelty. The object will tend to lose its novelty over time, and we'll tend to progressively lose interest in it up to the point where we go back to 'set point'.
Hint: look for mechanisms in your side projects to add novelty to the system so that your interest doesn't plateau.
When we built https://sametab.com as an internal side project, we used to constantly brainstorm new ideas about how this could eventually become. The more ideas we were founding the more the project looked new and appealing at our eyes.