There are likely as many reasons to write as there are writers. Some of us write for a living, some of us write to keep the demons at bay, and some of us write to impress someone else.
I write because the words have to come out. Writing is it's own reward. Write for you. Be selfish. If it doesn't satisfy you, how can you expect others to appreciate it?
What About Writing for Money?
I like what Stephen King said about doing it for the money (i've shortened the quote, but it keeps his intent):
…I never set a single word down on paper with the thought of being paid for it. I have written because it fulfilled me. I did it for the buzz. I did it for the pure joy of the thing. And if you can do it for joy, you can do it forever.
—Stephen King, On Writing pg. 249