I suppose there are as many answers to that question as there are writers, but if you can’t answer it for yourself, you probably have no business even trying. Why? Because the answer you give will shape the kind of writing you do, your tone, your approach to your work, and your willingness to persevere when the inevitable writer’s blocks, snags, and disappointments hit. The writer most likely to fizzle out or to write drivel is the one driven by extrinsic motivations: “publish or perish,” prestige, posturing to Support a Cause, fame, money. (Oddly, money is the least troublesome of these; a money-driven writer is often more hack than artist but at least has some motivation to stay on track and to produce something that an audience might actually want.) The writer who writes with genuine passion and interest and has a desire to share that passion and that interest with others is the one who is likely to stay the course, all other things being equal.
I write because I get grabbed by a story and find I can’t let go of it. I suppose I can be considered a historian of sorts, but my vehicle for exploring history is story; I want to know the feeling as much as the facts. It isn’t always a comfortable passion, and it’s certainly time-consuming; there’s always the itch to try to dig out one more fact, one more reminiscence, that might bring a person or a horse more to life. It certainly hasn’t made me rich or anything close to it—but it has made me a richer and deeper person, and helped me to connect with people who enjoy the tales I tell. I’ll take that. (Though more money would certainly be nice.)