So much about writing for a living involves patience. Lots of it. First off, it takes a great deal of mental discipline, concentration, and self-control in order to write. You have to shut out all distractions (not easy these days!), stay focused on a narrative throughline (in the case of fiction) or a topic (in the case of nonfiction or journalism). You even have to be "on" when you aren't writing, filtering through all the random thoughts and ideas that pop into your brain when you're housecleaning, or out jogging, or driving your car---and then keep track of which ideas are usable and which are not, and which can be filed away for possible future use. And if you're a features journalist like I am, you also have to "keep your finger on the pulse of America," as my editors like to say, so I can pitch them my monthly story topics that are timely and of interest to my publications' readership. Not an easy task, for sure.
But that's just the tip of the iceberg. Even when your writing is complete, you often spend a lot of time just waiting around. Waiting for interview subjects to return calls or to approve their quotes. Waiting for your editor to reply to your latest email. Waiting for your agent to get back to you on all those submissions she made to big publishing houses in the positively glacial world of New York publishing. Waiting for that quarterly royalty check. Waiting, waiting, waiting. And then more waiting.
I think the main reason most people who aspire to be professional writers simply don't have what it takes is a lack of patience. It often takes years and years of work, study, and self-defeating rejection just to get published, after all. And even after you've "made it," the waiting and the need for humility and reams of patience never really stops---if anything, it just gets worse. That's why when random half-committed (if that) people waltz up to me and say they want to do what I do for a living, I tell them, "No you don't. Go out and sell real estate, or get a job as a secretary or something. Don't do this." They usually give me a dirty look and walk away. But they always seem to take my advice, at least when it comes to getting a job.