I'm only going to speak for myself, but I must admit there are times that I wonder.
I received a quarterly report and royalties from one of my publishers--and it was just about enough for a nice dinner out for husband and me. Not much of a reward for all the time I put in writing.
On the good side, several of the books in that particular series were purchased--both paperback and e-books. That means people are interested in the series.
As for my true motivation--writing is what I do. Everyone knows I'm a writer and that's how I spend a great deal of my time. (Another batch is spent in promoting--but I wouldn't even sell as many books as I do if I didn't spend time promoting.)
Because I'm invested in my characters, I need to know what is going to happen to them next and the only way to do that is write the next book.
I love my writer friends--and I have lots of them. Ones I actually get to see in person--once a week with my writers' group, ever so often for the two Sisters in Crime groups where I actually attend meetings and events, and the writers I've met over the years and still am in touch with And I have a whole bunch of writer friends that I connect with on Facebook.
And last, but certainly not least, there are the readers who follow my series and let me know when they really like the latest book.
Simply, that's why I keep on writing.
Marilyn aka F. M. Meredith
P. S. It won't be too long before I'll be promoting another Deputy Tempe Crabtree mystery.