"That's a fascinating story. You ought to write a book about it."
A fair number of the clients who come to me for editing of a manuscript have had this experience. They've shared some part of their life story with others and gotten the above response more than a few times. So they decide to sit down for a few months and write out their story. Then they bring it to me to edit for publication.
And they generally do have a good story, often a riveting story. But they don't have a good book. Why? Because they don't know how to write. Well, they're literate. They know how to put pen to paper or fingers to keyboard and they can compose sentences and paragraphs. But they go blank when I ask them about writing courses they've taken, or workshops they've attended, or how extensively they've read in the genre they're working on. They go blank when I talk about the arc of the story, the development and motivation of their characters. How they chose their dialog stance. What creates a chapter for them.
What it boils down to is that they haven't done any of the work to become a writer. And it takes a lot of hard work to become a good writer. Not every good writer follows the same path. Some get an MFA in writing, some get a PhD in a related field. Some find inspiring teachers and great critique groups; some attend conferences and workshops and weekend retreats. Some do all their reading and writing practice on their own. But each of them recognizes that writing takes the dedication and discipline of craftsmanship and artistry to be worth disseminating.
It helps to have a great story, but to really be successful, that story needs a great author.