Unlike other professions, anyone can call themselves a writer. There's no license exam to take, no registration to secure, no certification needed. The good thing about that is anyone can become a writer and get into writing (and to a certain extent, who doesn't write per se?). The not-so-good thing about that is that the qualification is murky.
I mean professional writers tell you when you're meant to be a writer. The most common answer I've seen is needing to write as much as you need to eat or drink. That's well and good as far as self-examination is concerned but when can a person humbly call themselves a professional writer? Is it when they've had X number of works published? When they've written X number of fan fics? When they've won an award? When they're handed their first paycheck for writing?
Of course this is the insecure, wannabe-writer in me talking. But for most people who haven't been published or paid for their writing or won an award, there'll always be that voice whispering doubts.