In The Signification of the Phallus Jacques Lacan distinguishes desire from need and demand. Need is a biological instinct that is articulated in demand, yet demand has a double function, on one hand it articulates need and on the other acts as a demand for love.
So, even after the need articulated in demand is satisfied, the demand for love remains unsatisfied and this leftover is desire. For Lacan "desire is neither the appetite for satisfaction nor the demand for love, but the difference that results from the subtraction of the first from the second" . Desire then is the surplus produced by the articulation of need in demand (Dylan Evans).
Lacan adds that "desire begins to take shape in the margin in which demand becomes separated from need". Hence desire can never be satisfied, or as Slavoj Žižek puts it "desire's raison d'être is not to realize its goal, to find full satisfaction, but to reproduce itself as desire."
http://en.wikipedia.org/wiki/Lacan
