NY Magazine has a great collection of essays from a bunch of writers, performers, athletes, directors, and photographers about the incident or realization that made them answer their calling.
I wore that [cardboard Howdy Doody] magic set out. We had a dusty storage room in our little house, and I would sit in the dusty storage room in the afternoon and just work the things over and over again, for my own edification. I wasn't really performing for anybody. I was just obsessed with it. And incidentally, on my 60th birthday, my closest friend in the world found a copy of the Howdy Doody magic set on eBay, and it was advertised as "the same kind of magic set that got Teller started in his career." My friend gave it to me, and I just sort of collapsed and couldn't stop crying, because it was so thrilling. – Teller
I had worked on the melody in my bedroom in my parents' house, but I only knew about five guitar chords at the time. I think another realization I had was that even with rudimentary tools, I could enhance my world. Like suddenly I had a language to communicate my feelings, and that was also very powerful. I just remember hearing the guitar chords with the vocal melodies and having it sound like a semblance of something I'd heard, so it felt like I was joining a realm. – Carrie Brownstein
What I found out — what I discovered at City Paper — was that journalism is a done thing. In other words, there is no real "better than you." There's just the story you produced. That's what it is. Either you repeatedly asked questions or you didn't. But you made a choice. Someone else might be more curious than you, but the functionality of them being more curious than you is that they just asked more questions. That was a deep sort of lesson — that the winner is the person who keeps asking questions. That's the winner. – Ta-Nehisi Coates