"Acid wants the gleam, and the rust says it's only being corrosive."
-- late 19th and early 20th century Austrian writer Karl Kraus defending the nature of being a critic.
This is from a translation quoted by Jonathan Franzen in his recent piece for the Guardian in which he explains his skepticism about social media and other relatively new technologies. You really ought to read the whole thing. I already knew Franzen was a good writer, and I am now keen to check out Kraus' work too.
Oh, and the actual quote in its native form appears to be: "Die Säure will den Glanz, und der Rost sagt, sie sei nur zersetzend."