The point here is to move from "need" to "want".

If you "want" something, you're willing to pay a fair price for it (maybe), but you're also perfectly willing to walk away if the negotiations don't go to your liking. If the deal falls through, it's a disappointment perhaps, but no biggie.

If you "need" something, it's suddenly very hard to back away emotionally, and the other side will pick up on this and exploit it.