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." What did he mean by this?
The function of desire is to direct the subject into action, this action has no endpoint, if desire could ever truly be fulfilled it would be the utter logical collapse of the ego when further action is no longer required. Desire therefore is not aimed at completion but presupposes further desire before even the current object is even obtained.
Therefore a desire only reproduces itself as desire.
When we learned about this in high school our teacher used the Road Runner to explain what it is. Basically you desire to desire an object, not the object itself. Once you acquire the object you lose all desire and ego.