Simply, because I can. I've spent the entire day writing. I've added at least 14 pages to my newly revised manuscript. I absolutely LOVE my characters, even with their quirks and strange personality traits. Part of me wants to continue on during the night, but the sane part of me states that sleep is essential. I still have over 100 pages to add, but I'm not worried. I've got a great idea of what to add and where. I still have the final showdown scene to write (according to these characters in this particular setting) and I have a bunch of other minor areas to tweak. I LOVE my manuscript again! I can't believe I'm saying that after so long, but it's true. This couple is extremely special and this has been my first attempt to examine the characters' inner feelings and emotions in a story. I've delved into the characters' backstory before, but never have I played around so much with their emotions. It's truly a unique experience and it adds so much more to the story. I think I'm still a plot driven writer, but with practice I think I'll be able to weave in character evolution as well while the mystery/suspense unfolds.
I write because I have to. The stories build within me. The words demand to be written. I have no choice but to listen to the voices in my head. Most days I get crap for listening, but some days (like today) it makes all the other aggravation worthwhile. And as I type out the conversation my hero has with the heroine as he walks away from her in his stupid attempt to protect her, I think maybe, just maybe, this manuscript is "the one."