To use a curious javelina to teach kids about the southwestern U.S. (and prove that even the porcine can be huggable).
To create a new Christmas legend (and make sure everyone knows how termites get in and out of houses).
To tell a compelling love story by weaving together past and present, modern day Illinois and the Civil War South (and say it's OK to sleep a lot because dreams are powerful).
To prove there is at least one more terrific supernatural plot out there (and that there is a soggy ham sandwich-eating Harold Lovejoy in all of us).
To reveal what goes into a witches brew (or maybe that Burgoo in Peru).
And, to tell a tale of piracy and intrigue (and demonstrate that there's more to the Jersey Shore than MTV would have people believe).
Humor aside, I think while a book starts with an abstract idea or thesis, at some point the characters take on a life of their own. Kind of like Harry Potter and Edward Cullen. They're often talked about like they really exist. And my guess is that the reason that countless sequels to Pride and Prejudice have been written is because people can't bear to say goodbye to Mr. Darcy.
Writers write because even if millions of people haven't yet met our characters, they have become important to us. We want to know what happens to them.
Trager has said a couple of times that whole segments of dialogue come to her, and we've all talked about inspiration hitting us at random times of the day. How could we ignore it, especially once we've opened the door to a new world.
I'm not sure if I'm making sense, to be honest, but I will close by saying I continue to write because (like Ellen said) writing can be painstaking at times, but it is also fun. And I mean both the solitary work of putting words to paper and the social aspect of gathering with other writers. I have learned so much by hearing the stories others have to tell while they are still in the process of telling them.
(Hopefully, the other members of the Write 6 will forgive me for summarizing, as I did, their own terrific works in progress, which I greatly respect.)