It's an interesting question. And one with more than one answer.
First of all, I'm a writer. I've always been a writer but I've not always been writing. I've spent half a lifetime excusing myself from writing on one spurious basis or another. The most prevalent has been the myth that I need a long stretch of undisturbed peace and quiet in front of me before I can even contemplate putting pen to paper. This just isn't true and is a construct of the part of me that feels threatened by the wonderful peace and satisfaction that writing brings to my life.
Other excuses include having to have a favourite pen and the right notebook for it to work. Or having lived an interesting enough life to be able to write something interesting. And so on, you're getting the picture, I'm sure.
I suppose the above is more of an explanation of why I don't write as much as I would like.
I like the feeling I get while I'm writing which also lingers on as a nice afterglow. (Reason 1)
I enjoy telling a story. (Reason 2)
I get a lot of satisfaction from the way my plots form themselves and make sense, even to me. (Reason 3)
It gives me a thrill when someone buys one of my books. (Reason 4)
It makes me proud when people tell me how much they've enjoyed reading my work. (Reason 5)
I know that the more I write, the better I get at it. Since 2007, when I really began to write on a regular basis, I have completed three science fiction novels which weren't really good enough to publish but taught me a hell of a lot about myself and the craft of writing. Three published crime thrillers ( the Jenny Parker series) which are published and selling very well. The latest of these, Limited Liability, is by far the best of the three but that's because I am learning all the time. The first two are pretty good themselves, it's just the way that Limited Liability flowed onto the paper that I really like.
I had expected to be done with Jenny Parker after Limited Liability but I have had enough expressed disappointment about it being the final installment that I caved in and wrote a fourth, Critical Analysis, which is at the editing stage. now and should be published later on this year. I find it takes about half as long to go through the editing process as I take to write the first draft. It also depends on the availability of my editor, who is always very busy because she's so good.
Then I'm collaborating on a joint project with an Irish writer called Daragh O'Reilly. He's a comic genius and I'm learning a lot from him. He's written a colonic irrigation scene that's pure poetry!
My latest project involves a new character called Tyrant. It's coming along very well. Just so you know, it's not a crime thriller.