on Jane Friedman site:

When I ask my clients why they want to write a book, they will often start by giving a simple answer: “I want to share what I have learned” or “I don’t want other people to suffer like I did.” These answers are part of the truth, but they often shield deeper reasons. These reasons, this deeper why, form the core of your motivation and momentum; you’ll draw on these reasons when you feel despair or imposter syndrome.

If you never ask yourself why you are writing, you are far more likely to write in circles, fall into frustration and doubt, and come to believe that writing depends on some elusive muse or a series of special habits (e.g., write 1500 words a day, write for an hour every day, write when the full moon is waning) rather than deep self-reflection, discipline, and persistence.

Identifying your why first has an enormous impact on your capacity to both write and complete a book that resonates with your desired reader. It’s often the difference between writing a book that people want to read and either (a) never finishing, or (b) finishing, but writing something that is so watered down and wishy-washy that it fails to make an impact.

You can write your way to an answer—absolutely. I have done it, and writers I know have done it, and we have all heard of famous writers who have done it, but the truth is that for most of us most of the time, it’s wildly inefficient, ineffective, painful, and unnecessary. That’s why we start with why.