"Why I write" is of course an unanswerable question. Writing is an activity where matters from one's deepest interior are communicated directly to the outside without being completely processed by the conscious mind. The weaving of stories or essays by a writer and of paranoid fantasies by a schizophrenic have much in common. Writing is a compulsive activity and no-one can fully explain his reasons for doing it. Here are some possibilities.
Defiance
Writing is an act of defiance, of people who think you can't or don't want you to. Writing is always inconvenient to somebody. The fact of the writing itself frequently unsettles people even if they haven't read it. The content may express ideas which distress readers, or they may get the "Mr. Jones" feeling (there's something happening here, but they don't know what it is.) Even when the reader agrees with the underlying statement, he may feel it was better left unsaid. Since daily life involves a substantial web of consensual deceptions, clear, logical writing may expose commonly held falsehoods in an inconvenient way.
The best revenge on an inconsiderate lout is to portray him somewhere in your writing, as an example given in passing (don't let him overwhelm the theme.) Whether or not he understands the reference is to him, you have won.
When an essay of mine is distributed on the Internet and I get sincere but very upset email from someone who spent a lot of time thinking about it, I feel I have done well.
Writing is also an act of defiance of the Second Law of Thermodynamics. Once it has been severed from any single physical form, especially if it has been propagated on the Internet, writing is immortal in that it cannot be destroyed by any one act (save one that destroyed the human race.)
Action
Writing is a form of action. If you hold a beautiful idea up in public, people will attach themselves to it. If your idea is powerful enough, keep holding it up, for years and years. Good ideas tend to win in the end. Martin Luther King said, "The arc of history bends towards justice."
Writing is also the evasion of action. Those who write feel they have done enough, and dispense themselves from doing. There is no form of words which makes as powerful a statement as that of placing your body in danger to communicate an idea. Gandhi said, "We must be the change we wish to see in the world." He didn't say, we must write about it.
Engineering
Writing is engineering. You join the ends of ideas together in a structure which has utility and also beauty. If you know you have done a good job, it doesn't matter if anyone else knows.
Dialog
Writing of any kind, fiction or not, is a dialog with yourself. When you weave ideas together, or narrate the results of your characters' actions, you have answered a question you asked yourself, though you may not know what it was.
Shield
Writing protects us from life. If one writes too much, it is likely he lives too little. "Tout sa virilite passait en reve," Zola said of a character who spent his time dreaming ("all his virility passed away in dreams"). Writing is a virtual reality engine in which you can endlessly play out the consequences of choices, or create images of the world as it would have been if the things you desired came to you. If you write enough, you may never need to make the choices you are imagining. Enough writing, intensely carried on, may stand as a consolation prize for the things you have lost. Yeats wrote, "And soon enough the dream had all my thought and love/ and not the things it was an emblem of."
Escape
Similarly, writing enables us to pretend we are the people we want to be. Up to a point, writing may enable us to discover what we want, so we can take action to attain it. "If you will it, it is not a dream," Herzl said. If you hold up an idea long enough, and are willing to endure the consequences of doing so, then the act of portraying yourself as the person you want to be may turn you in to that person. This is the exception, not the rule. Beyond a certain point, writing dispenses us from changing; we imagine that we have become, or already were, what we wrote about. Whenever we read about the life of a writer, we usually discover that he was pettier, crueler and more vain than his protagonist.
Beauty
Writing enables us to be beautiful whether or not we are in life. Jean Rhys is dead; I have no idea what she looked like, young or old. I do not know if she was kind, tender, or funny. But yesterday I read the following lines, written more than sixty years ago in Voyage in the Dark:
It was one of those days when you can see the ghosts of all the other lovely days. You drink a bit and watch the ghosts of all the lovely days that have ever been from behind a glass.
It was as if I had just met Jean Rhys, and was thinking: "I'm going to fall in love with her. I know I shouldn't do it, because she's going to be trouble. But I'm going to anyway." And at the same time I thought that if I could just write one or two phrases like that one, I'd be very happy.