Sex
"The look", Sartre explains, is the basis for sexual desire; Sartre declares that there is not a biological motivation for sex. Instead, "double reciprocal incarnation", is a form of mutual awareness which Sartre takes to be at the heart of the sexual experience. This involves the mutual recognition of subjectivity of some sort, as Sartre describes: "I make myself flesh in order to impel the Other to realize for herself and for me her own flesh. My caress causes my flesh to be born for me insofar as it is for the Other flesh causing her to be born as flesh."
Even in sex (perhaps especially in sex), men and women are haunted by a state in which consciousness and bodily being would be in perfect harmony, with desire satisfied. Such a state, however, can never be. We try to bring the beloved's consciousness to the surface of their body by use of magical acts performed, gestures (kisses, desires, etc.). But at the moment of orgasm the illusion is ended and we return to ourselves, just as it is ended when the skier comes to the foot of the mountain or when the commodity that once we desired loses its glow upon our purchase of it. There will be, for Sartre, no such moment of completion because "man is a useless passion" to be the ens causa sui, the God of the ontological proof.
Read more about this topic: Pour Soi, Overview, Part 3, Chapter 1: The Look