The terrific people over at National Novel Writing Month have a forum where writers posted their answers to the prompt: "I know that I'm a writer because..."
Here's my answer.
I Know that I’m a Writer Because….
It’s the only thing that I have ever returned to, the thing that nags at me, that compels me. Even when I’m not actually putting pen to paper or fingers to keyboard, I’m composing scenes and scenarios in my head. When I was in elementary school, I was often “caught” daydreaming, staring out the windows looking at the trees. I wasn’t daydreaming. I was writing stories. On the playground playing hopscotch with friends, I created elaborate love stories for each of us with our favorite TV and movie stars serving as our heroes with the happily ever after. By junior high, I was writing episodes of my favorite television shows. When I got to college, I was going to be a psychology major, but I took one look at all of those science and math courses and asked, “Where is the department where people write?” My heart and soul knew who and what I was, even if my head had not yet figured it out. I’ve spent my life writing. It’s what I am, who I am and why I am.
Now, post below your answer to the prompt, and be sure to take a look at all of the ways other folks in the NaNoWriMo Forum know that they are writers.