Because you can't take it back. Once you write it down, it's real, it's an extension of you. Whether you think about it beforehand or not, it becomes the way people see you. Sure, there is the publishable stuff, the perfected pieces you intend for others to see and fervently hope they will like. But there is a lot of stuff that should just never be read by others. Especially in this day of social media and digital communication, for instance, you should give an extra little thought to what your comments might look, sound, and feel like to someone else before you go to typing them down and pressing Enter or clicking Post.
Just like there are some things you shouldn't say, like "You look fat in those pants," there are some things you shouldn't write.
But my all-time favorite "you look fat in those pants" writing debacle happened in 9th grade geometry class, in which my teacher confiscated a note I was writing to a friend, in which I may or may not have mentioned how much I hated said teacher because of her annoying personality and poor teaching skills, which I then outlined in great detail.
Life's lessons are hard learned. I learned quickly that even grown-ups, well, especially grown-ups, could not be trusted. I have not written one, no not one, journal entry in almost 15 years. And I only share the good poetry, and then only with people who get poetry.
And I learned that when you insult someone, even if it was meant to be private and even if they fully deserve it, they will strike back at you and "read" your note to the class (proving that you were correct in your original analysis) and basically indicate that you are in love with Joe Watson, who is sitting across the room.
That's the key here. If you write things down, no matter where or for whom, it will not stay private. So be careful. You have been warned.