A number of years ago, when I was doing a lot of freelance work, I had numerous requests for my writing services. Some were from students who were looking for someone to write their paper for them. Others legitimately needed my assistance with writing and editing. And then there were those who hinged their retirement plan on writing a best selling book and living off the royalties forever.
One person in particular emailed me and wanted me to write her story pro bono. She was in dire straits and needed money, and she was sure her story would net her an income, which she desperately needed because she was unable to work due to an injury.
This is what a lot of people don't understand: Because you have an interesting story to tell doesn't automatically guarantee a publishing contract. The writing of the story is a long process. Lots of time energy, blood, sweat, and tears goes into writing a book. Writers paint a picture with words so the reader can use their imagination and understand exactly what the writer is writing about. Clearly, this can be very challenging.
Also, no writer should work for free. Your hairstylist, your dentist, your plumber ... they all don't work for free. Unless you are collaborating with someone, you should be paid for your efforts.
The biggest thing of all is that the book may not make money, especially if the author has no desire to market or promote it.
So, what are you writing for? When you write because you have a passion for it, a drive for it, then it will show in your writing, and your readers will know you are writing from your authentic self.
If you are writing only because you think you will be offered a 6-figure advance, then you are writing for the wrong reasons.