You must be passionate. About your life, about your loves and about your words. If you’re not passionate, the words will never jump off the page and keep your readers awake until their eyes burn and they yearn for sleep ‘after just one more page.’
Not every writer is going to be successful at this endeavor. Some will write perfectly acceptable books; books which sell well and have an audience. But the novels where the author is passionate; oh, those are the great ones. Those are the ones where the reader will buy everything you write because you give them a world to believe in and characters to embrace.
The authors I love who fall into that category, regardless of genre:
Marion Zimmer Bradley (fantasy), Mercedes Lackey (fantasy), Chelsea Quinn Yarbro (fiction), Robert Asprin (fantasy), Kim Harrison (fantasy), Lee Child (mystery), Steve Hamilton (mystery), Robert Jordan (fantasy), David Eddings (fantasy).
I’m enough of a reader that I’m sure there are tons more I’ll think of as soon as I end this post, but so be it.