What kind of book will you write?

This thought came to me today, I see a lot of people on twitter and in
the blogosphere becoming first time authors, even my neighbour is a
famous Australian expat blogger and has published a book, which i read
in one go and enjoyed immensely. I enjoy a good story so I am curious about how these first time
authors get the impetus to write? What goes inside their heads? I
would like to know what these writers want to expresss,

Is it the cognitive dissonance in people they are trying to express by
their story and hence are able to capture the pulse of their readers?
(Chetan Bhagat) OR

Is it just about a fantasy which is fictionalised and wherein hidden
and perverted desires & painful experiences expressed, which would
have never have found expression in their real lives, As most of the
first time authors will tell you their first book has been partially
autobiographical. (Arunadhati Roy) OR

Is It an intellectual pursuit of word play and the power to take the
readers into another realm or period or place, as a means of escapism
for the readers from their mundane lives into magical worlds of words
and thus impress upon the reader the skills of the author and hold
them in sway till the end of the book by their sheer karishma and
verbal/written skills? (Salman Rushdie/ Amitav Ghosh) I am eager to meet some writers/storytellers/screenplay writers to
understand the writing process.