through fantasy, we learn how to desire

Our desires rely on lack, since fantasy, by definition, does not correspond to anything in the real. Our object of desire (what Lacan terms the "objet petit a") is a way for us to establish coordinates for our own desire. At the heart of desire is a misregognition of fullness where there is really nothing but a screen for our own narcissistic projections. It is that lack at the heart of desire that ensures we continue to desire.

Because desire is articulated through fantasy, it is driven to some extent by its own impossibility.

The act of sex for humans is so much caught up in our fantasies (our idealized images of both ourselves and our sexual partners) that it is ultimately narcissistic. As Lacan puts it, "That's what love is. It's one's own ego that one loves in love, one's own ego made real on the imaginary level"