Maybe I would have been a rockstar, I know at some point I wanted to play the guitar and be in a band. Or do something adventurous and humanitarian like be a philanthropist. I could see myself trekking through the Amazon jungles, looking for exotic plants to cure the common cold. But I'm not. I'm sitting on the floor of my bedroom thinking about my characters even as I type this.
I am a writer.
First, we decide to write.
Then, we decide to keep doing it.
I decided to write when I was nine and sat down to pen my first story. I decided again when I was fifteen and had about 95 half-completed stories under my belt. It's that second decision that counted the most, it's what got me through my first completed novel, as crappy as it was it was done and it was mine.
That second decision drives me everyday. It made me finish a second book and start a third. It keeps me going when I'm sitting in the dark, staring at a blinking cursor with no idea of how to make it move.
With that said, I believe that writing chose me, as it chose you, Other Writer. The ideas come--in the early hours of the morning when we're half asleep, they come during a phone call from a friend asking us to hang out--most of the time when we want nothing to do with them. We don't ask to be nagged by persistent characters caught in love triangles and century old curses, we don't choose that.
What we do choose is the part where we roll out of bed and stomp to the computer before we've even had coffee, where we tell our friends "not today, I'm writing." This is the choice we make, even as we're thinking "Good God, man! Couldn't this damn idea wait until after my alarm went off/after dinner with *insert friend's name* ? They were gonna pay!!"
The writing chooses us, but we have to make the choice to choose it back.