Writing a novel is a long-term commitment, and an author has to really enjoy the novel he or she is working on. Why? Because they'll be thinking about it a lot, re-reading it a dozen times, and revising it probably at least a dozen times (and a few more while working with an editor, if the novel finds a publisher).
The process of writing a novel will consume vast expanses of time, inevitably crowding out other enjoyable activities. If writing the novel isn't made a priority, it will never get finished. Writing doesn't necessarily have to be the top priority, but it can't hover near the bottom.
I think most people feel they have at least one novel in them. Maybe they do, but very few ever get it out of them--and even fewer, successfully. Why? Because while there is enjoyment in writing, it is also work, hard work. It requires practice and study and close attention to detail--in addition to the wordprocessing.
And there is the knowledge while writing the novel that, at the end of the day, there is no guarantee of success--that it'll find a publisher. The competition is stiff, and that's an understatement. And even if the novel does find a solid publisher (or the author decides instead to self-publish), that it'll find readers that enjoy the story labored over for so long isn't a sure thing--far from it.