I will quote from a book I just bought. It's for authors who want to write their first novel:
Why are you writing? There are hundreds of easier ways to make money. Thousands of better methods by which to self-actualize. Millions of less taxing ways to entertain oneself. You should write because you can't not write. You should write because some stray scrap of your soul is trying to manifest itself verbally. ... You should write because even though the process terrifies you, the absence of that process terrifies you more. Which is to say, you have to love it. Unconditional love, and often, unrequited. There is no guarantee it will love you back.
I've written almost every day since I was thirteen years old. In high school, I painstakingly wrote longhand in narrow-ruled journals with floral covers.
At fourteen, I decided my thirteen-year-old writing was so immature and poorly written, and I tried to burn the journals. It's actually pretty hard to burn a thick wad of paper. I finally just threw them into the trash. For the next year, I periodically suffered moments of horror when I imagined that a stranger would come upon them in a landfill and read my secrets.
I still write into electronic journals daily. When I go to a place without a computer (like Burning Man), I carry a paper journal.
Writing for an audience is different from writing for myself. It requires more thinking -- tweaking of sentence structure, endings, general flow.
Blogging = writing for an audience.
Occasionally on other blogs, the author will drop a mention of "my other more personal blog" or "some other blogs I write for myself", and I know they blog for the same reason as I do -- because we are compulsive writers.