Because Your Mind Is A Terrible Thing.

Best argument for getting a second set of eyes on your MS, courtesy of Mark Twain:

"You think you are reading proof, whereas you are merely reading your own mind; your statement of the thing is full of holes & vacancies but you don't know it, because you are filling them from your mind as you go along. Sometimes--but not often enough--the printer's proof-reader saves you--& offends you--with this cold sign in the margin: (?) & you search the passage & find that the insulter is right--it doesn't say what you thought it did: the gas-fixtures are there, but you didn't light the jets."

Damned pesky jets.