"However," replied the universe, "The fact has not created in me a sense of obligation."

“Here is another thing to remember every time you sit down at the keyboard: a little sign that says, ‘Nobody has to read this crap’. You are not writing to impress the scientist you have just interviewed, nor the professor who got you through your degree, nor the editor who foolishly turned you down, nor the rather dishy person you just met at a party and told you were a writer. Or even your mother. You are writing to impress someone hanging from a strap in the Tube between Parson’s Green and Putney, who, given a chance, will stop reading in a fifth of a second. So the first sentence you write will be the most importance sentence in your life, and so will the second, and the third. This is because, although you may feel compelled to write, nobody has ever felt obliged to read.”

-Tim Radford via Ed Yong.

(Also thanks Stephen Crane and vozamer.)