So WHY...Why write? I won't say too much because I've already addressed the topic here and here.
Most writers seem to be driven to write for some reason. I think I've mentioned that for myself, I just like to tell a story. At work I met someone today who writes screenplays (!) with a writing partner and evidently even made a documentary! People have encouraged her to write novels or non-fiction, but her passion is screenplays, and from what she said, it sounded like she'd probably write without an audience to read it.
I try to be the same way...before the Internet was big (and before I was working full-time!), I was definitely the same way. Now I find it's too easy to just read and post to messageboards and Facebook, and get my writing bug out at work (I spend all day writing sternly-worded letters) instead of writing fiction for fun. I do still write fiction, of course, but not as much as I used to. Some of the fun does drain away (for me, anyway) when you actually have people critique it...
Still, as my dad likes to say, "Writers write." While I'd like to make a living off my writing, mainly I'm just driven to do it, and I like to think led to, as well.
But other WHYs.
A big one I hear from people who critique me seems to be "Why are your characters doing that?" I've had compliments on my characters, yet sometimes they seem to act to advance the plot more than doing what comes logically. I'm not sure how to prevent that, but I think the only way to SPOT it is to get some third party to look at my work. I just don't see it when it happens.
Which explains WHY you need someone to critique or give some sort of feedback on your work! Even if it's not fun! (That someone can be an editor, critique partner, beta reader, or even WRITING partner, but if you're just writing on your own without any feedback, your fiction will probably be a lot weaker than it could've been.)
One more WHY would be the overarching message of your work/s. When I was younger, I just wanted to write, yet my first full-length fantasy novel (high school; not so good) literally had a Jesus figure show up. My dad charitably commented that he'd read books worse than mine, and that it got awfully religious at the end. LOL. So on the one hand, I just want to write. But on the other hand, God seems to creep in anyway. The majority of the things I've written have had substantial express Christian content and much of the rest has had at least some in there.
I think whatever the writer believes will probably seep through into his or her work, really, whether those beliefs are the motivation for the work or just something that slips in unbidden.