Sarah Hoyt just put up her own post talking about why she writes. I figured it was time for me to do it again (I've discussed it, but in other locations or as part of other subjects).
The short form? I write because it's what I do.
I've been writing stories, and other things, since I was six. I haven't stopped writing since, although various things in life can slow down my production and exactly WHAT I write can vary.
To give you an idea, in the past couple of years I have written five full novels, only ONE of which (Portal) was under contract when I wrote it. I have in addition started three other novels, and written a fair amount of things like backstory. This is, one should note, in addition to hundreds of thousands of words of LJ entries, Usenet postings, and the HUGE amount of writing I do for my day job; in that same period of time I have probably written over 40 Phase I and 6 Phase II proposals, final reports for a dozen projects, and interim reports for five or six projects every month or two.
There are some things -- especially in the days when I was not being paid to do any of this -- that can divert or sublimate the need to write stories; roleplaying games, for instance. But even there I end up writing; I had one character that had 40 pages of backstory before I even started PLAYING her.
For me, writing is. put bluntly, FUN. I read what other authors say about writing and in many cases I can't get my head around what they're DOING to make it NOT fun. Getting time to write is the biggest challenge, since (as mentioned above) I have a day job. I also have a fairly large family (wife, four children) and there are family and friend social obligations (unwise to miss your mother in law's birthday, for instance).
Oh, there are occasional elements that aren't all beds of roses; writing hard SF is probably the most WORK-like, because people who READ hard SF like to be picky about all that real-world physics stuff, and you need to make sure you show your work enough to convince them that you're not just pulling stuff out of your nether regions. So it can certainly be a pain in those nether regions to have to (for example) figure out just how nasty the radiation is around Jupiter, and how you might manage to protect people from that radiation, or devise some form of propulsion that will meet the story demands without violating the laws of physics. Even if you're going to ignore some aspects of reality for the story, you have to figure out how to smooth over that little problem. But even the most DIFFICULT research I've had to do (and fairly expensive on occasion, when you have to buy pricey books) is no more annoying than the work I have to do for a technical proposal, and since *I* have gotten to define exactly what I want to write about, the research is usually fairly fun in and of itself because I learn something about things I am, at least theoretically, already interested in.
And the actual WRITING? Fun. LOTS of fun. Once I get my head set in the universe in question, I fly through it at about 1200 words per hour (a rate that hasn't changed much over the seven years I've been tracking it). I get to SEE what happens in the story (even if I've been forced to outline, the outline doesn't really tell me everything, and often I diverge rather drastically from the outline once I start writing). I get to chuckle over lines I find funny, thrill to the confrontations I've anticipated, and make REAL things I've only before had in my head.
I don't seem to often DO much of the agonizing I see from other authors. Partly it's that I just don't do "drafts", really. I don't rewrite things unless a VERY large amount of time has passed since the first time I wrote it (and thus my style and/or the universe has changed enough to demand it), or someone (generally, of course, an editor) has pointed out something that needs to be changed.
I don't have to batter my head against the plot very often; I know where I'm GOING and that's pretty much enough, the details will work themselves out. The important thing for me is to KNOW THE UNIVERSE. If I know the world the characters live in, understand its rules (both physical and dramatic), then answering the question "Okay, just WHY is it so important he has a goldfish before they leave?" will fall naturally out of the universe and the overall plot goal. (In fact, the most frustrating thing in writing a novel is that I want to get to the end RIGHT NOW and SEE THE FINALE and I have to write the other 90,000 words to GET there.)
Another part, though, is that I don't concern myself with what other people will think when reading it, at least not much. Don't get me wrong: I love having readers. I love getting feedback on what I've written. I love seeing my books on shelves, knowing that someone's willing to actually PAY MONEY to read what I've written.
But I'm not writing for anyone but me. Certainly a publisher expressing an interest can FOCUS my writing attention on a particular thing, but whatever I focus on will still be something I'm happy to write for myself. I don't, however, worry about whether what I do is going to work for some particular audience, because I'm writing for the audience that wants to read the kind of stuff *I* want to read, but that isn't OUT there for me to read, and I'm writing to get the things I have in my head out onto paper (or electrons) so I can look at it better.
Generally I'm not writing for anything other than "telling a good story", either, so the only part of me that's bound up in the work is the part that's having fun -- and that a critic can't really take away. Yes, a bad review stings for a moment, but it doesn't have any real bearing on the work itself. I *did* deliberately write Grand Central Arena as a tribute to Doc Smith, so in that sense I was also writing for another purpose... but the purpose was also to try to recapture the same sensawunda I got from his books.
So in the end, I write because I'm a writer. I write because it's fun. I write because, as Heinlein said, it sure beats honest work. (Alas, as of yet it has not allowed me to ESCAPE the honest work, but I'm still hopeful)