I really can't think of any other feeling that compares to that of writing something new.
I write for many reasons, as I'm sure we all do. However, for me, the idea that I can pull any number of thousands of words from my mind and place them together like a puzzle to end up with something that has never existed before is truly awesome.
Just knowing that I've created something new is reason enough; if what I've created, however, means anything to someone else or affects others in any way, well, that's really just icing on the cake; but mainly, I write for myself.
I don't think anyone can deny that feeling of pure accomplishment after writing something that you are proud of. It has nothing to do with ego, its simply an undeniable feeling of 'Yes! I've done it!"
My life, thus far, isn't full of great accomplishments. I'm not good at sports, I couldn't draw to save my life, and I've never really done anything of any noteworthy significance. But I can write.
My writing is my accomplishment. And everytime I do it, I'm reminded of that. Everytime a friend or family member asks me how the writing process is going, or when they'll be able to read my book, I know why I do it. Even those stubborn writer's block moments, or those 'middle of the night' moments, jumping out of bed to scrawl something on a notepad (I now keep one on my night-table) or the times when hours are spent looking for just the right word...I know all of that is totally worth it. Because at the end, when I read what I've written, I get to be proud that it's mine.
But that's me; why do you write?