Writing can be dangerous. I learned this the hard way as a teenager. I confided the good, the bad, and the ugly in my diary, only to have said diary discovered. This wasn't all bad...I was making some pretty stupid choices at the time, and the revelation of these choices helped my family set me straight. Ever since that time, however, I've been cautious about what I put in print.
Writing doesn't become any less risky with adulthood. True, no parent will ground you for your confessed misdeeds. But the stakes are higher. The wrong word used, the wrong blog posted, the wrong opinion expressed, and you may find yourself out of a job, unfriended, or misunderstood by others. Publish your writing, and you better be very, very careful not to libel, or plagiarize, or otherwise place yourself in a legally compromising position.
But still I write. I came across a great quote this evening by E.M. Forster: "How do I know what I think until I see what I say?" That's it, then. I write for clarity. I write to streamline things, to try and make sense of the seeming chaos of my brain.
I used to think I wrote to answer questions, but I find that the more I write, the more questions are raised.