Many writers handicap themselves by writing for external reasons. Far too often, people decide to write a novel in hopes of achieving fame or fortune, or to impress friends and family, or simply for the ego boost and bragging rights. However, people who pursue money or celebrity rarely end up with books published through legitimate publishing houses.
The idea of writing a novel is a romantic one. People imagine the words just flowing out of their fingertips and the story translating easily to paper. They do not consider that an average novel can be 75,000 words or more. While the first few pages may come easily, most writers will eventually hit a wall - especially with their first novel. External motivation may not be enough to push through and complete a rough draft.
Others do not understand that the writing process involves far more than getting a first draft down on paper. Rewriting and revising are stumbling blocks for many people who do not have personal reasons for writing their story. It isn't just about fixing grammar and style problems - many stories are almost completely rewritten from the first draft to the second.
For every JK Rowling or Steven King, there are hundreds of other published authors who still have to work their day job, not to mention the scads of potential authors who have not managed to get their work published. Submitting a final draft to agents and publishing houses can be a frightening process, and one that requires a thick skin. Many of the most famous and popular writers out there have stacks of rejection letters to prove the difficulty of attracting attention. You must have confidence in yourself and your writing. It is easy to be discouraged.
The best reason (and I would argue the only reason) to write fiction is because you love it. If writing is a passion and something that you feel you have to do, then you should write your novel. If you have a story that is bursting to get out, then write your novel. If you are tenacious and thrive on hard work, then write your novel.
When a writer loves what they do, it is evident in their story. The characters are vivid and the dialogue pops. It is obvious that they have studied their genre and enjoy reading it for fun. The story is filled with passion and life.
Like all rules of writing, there will be exceptions. There are working writers out there who have immense talent but lack the passion, or who focus on the paycheck rather than telling a story. However, these exceptions are few and far between.
Let the success stories be inspirational, but write for yourself first!