This is both the easiest and hardest question in the world to answer. The truth of the matter is, I write because I have to. That's why the question is easy, but also why it's hard. To someone who doesn't have this same urge, it's hard to explain exactly what it is that makes me need to write.
If I had to, I could probably go a week, maybe two at the outside, without adding words to a story, but after that it starts to become harder. I have a compulsion to write the same way some people might be compelled to watch their favorite TV show.
So, why do I write? You could probably select any of the why-I-write.tumblr.com images at random and I'd be able to sympathize. Let's have a look at just a couple:
For me, personally, writing is cathartic. It helps to relieve stress or to celebrate a win. If I have a busy day at work, there's nothing better than coming home, switching off and letting the characters take control. If something great happens, after celebrating with my family and friends, I want to celebrate with my characters.
I've always loved the idea of walking in someone else's shoes though, when I was younger this was expressed through a love of the stage (both dance and drama) where with a simple costume and a different choice of words, you could be transformed into another person. I guess for me, writing is very similar. I can imagine this person who is my character and follow the way they handle a situation. As I've probably mentioned before, this is often not the way I would handle it, but there's something fun about seeing someone react in a way you would never expect.
There is probably a selfish part of me too that wants to leave a mark on this world. In my day job, I am important day by day, but would anyone really remember me in ten years time? Twenty? Will what I do now have any impact on the next generation at all? I have my family and friends, and I know that they'll always hold me in their hearts, as I hold them in mine, but if I look deep enough inside, I'm certain there is a part that wants to write something that will endure into the future.
What's the biggest goal I have for my writing? Besides getting a shiny, physical book with my name on it (which is ultimately my biggest goal) it would have to be connecting with the reader and making them feel something for the characters who I've become so intimately familiar with. I long for the sort of dynamic where readers will argue passionately for and against choices my characters made, where they will want to know, "what happens next." For people to care.