It's funny that another Jack Black movie, "The School of Rock" (2003), has a similar message: Do what you love because you love it. Even if you are not very good at it, you can find a way to fit it into your life. https://moviewise.wordpress.com/2013/03/05/the-school-of-roc...
Another movie that shows this lesson is the 2013 documentary "Salinger": Be driven by making your work great, not by the reactions of others. https://moviewise.wordpress.com/2015/08/03/salinger/
Which is influenced by Hinduism, "you should do your work as perfectly as you possibly can with no thought of rewards, and only that way can you be a really happy person.”
It's about being able to identify what you can and what you cannot control, focusing on the can control and just letting the rest occur. This is something I struggle with.
I see things, constantly, that are just rage inducing in terms of how stupid they are and how the problems they cause impact my life negatively. And I can't do anything about them. Yet I still spend time being negative, anxious, or otherwise focusing on them. It doesn't help, the problems still happen, and I spent time ruminating about them.
I think the frustration often arise when we create the illusion of control and unexpected events shatter that idea, bringing ourselves in denial.
A good measure is to keep a buffer for unexpected events and prepare yourself sentimentally.