What makes a quote quotable?

The forceps of our minds are clumsy forceps, and crush the truth a little in taking hold of it.
- HG Wells

It is well-known that in Casablanca, Ingrid Bergman never said: "Play it again, Sam"; she said, "Play it, Sam." In this week's anniversary edition of the New Yorker Louis Menand says, "...quotable quotes are coins rubbed smooth by circulation", and the most famous quotes, as originally uttered, were not quotable quotes; they needed some "editorial attention". Editorialization ensures that the quote survives and consequently the person who said does too. Quotes are memes that have a life of their own.

So what is a quote anyway? Why do we find them magical? The last paragraph of his essay is just as quotable for the reasons he describes:

Public circulation is what renders something a quotation. It’s quotable because it’s been quoted, and its having been quoted gives it authority. Quotations are prostheses... We pick them up off the public street, but we put them to private uses. We hoard quotations like amulets. They are charms against chaos, secret mantras for dark times, strings that vibrate forever in defiance of the laws of time and space. That they may be opaque or banal to everyone else is what makes them precious: they aren’t supposed to work for everybody. They’re there to work for us. Some are little generational badges of identity. Some just seem to pop up on a million occasions. Some are razors. "I see a red door and I want it painted black." "Devenir immortelle, et puis, mourir." "Much smaller piece." "You’re two tents." The quotation I have found most potent in warding off evil spirits is the motto of the Flemish philosopher Arnold Geulincx (1624-69): "Ubi nihil vales, ibi nihil velis." "Where you are worth nothing, you should want nothing." That’s mine. You can’t use it.