At different times in my life I have written for very different reasons. Now I write all sorts of things for all sorts of reasons, with many levels and types of motivation. I need to ask myself why I would put my time and energy into writing a novel. Because while I have packaged this choice for myself and perhaps others as trading in scholarship for imagination, the fact is that I could just stop doing scholarship and not replace it with any kind of writing. My decision to give up scholarly writing went into effect (a) roughly after I was promoted, and (b) specifically after my last prior commitment was satisfied, in Istanbul. This would lead me to believe that the main reason for engaging in this activity was to achieve the promotion, putting aside what that might mean to me. (Although probably I ought to take up that question ...)
The desire to be involved in creative writing comes from a sense that it sustains my life: years of scholarly pursuits had left me feeling dishonest, inept, and pointless. To write and think imaginatively is simply necessary for me to regain a fuller experience of life, and avoid alcoholism or dissipation. Or so it seems to me sometimes. I began to feel this last spring when I was teaching poetry and fiction to my Slovak students: which could be a clue as to how to sustain greater interest in teaching.
My ambition is to write something really good and to be recognized for having done so. My ambition is to publish a work that people will want to read, and that they will pay money to read, to have a book with my name on it on the shelf. My ambition, in the end, is to make a little money out of writing so I can retire from my "day job" at around 62. I am not a youngster, which means that I'm not just starting out as a writer, and that I don't have forever to develop my talent. The time is now -- relatively speaking --- to express what talent and experience I have. Part of my experience and wisdom ought to consist in patience and taking the necessary time to do something well. There are practical reasons for this also: bring something good to market the first time and the prospects for success are better. Being more practical yet, if I were planning to write a trilogy, for instance, plan the whole thing thoroughly, create the world thoroughly, do the research, up front. That doesn't mean waiting a year before writing a single word, but it means having a fully conceived notion before getting in too deep. This was how LOTR, Harry Potter, and HDM (Pullman) were written, as I expect is true of many other very long books divided into three or more or fewer parts. 2666 presents as five separate, semi-free-standing novels, but in fact when read together they form a whole.
To commit to a whole of that scope is daunting, but as I always say, in for a dime, in for a dollar.