Psychologically, the regressive longing for the security of childhood and early youth.
Jung interpreted incest images in dreams and fantasies not concretely but symbolically, as indicating the need for a new adaptation more in accord with the instincts. (This differed so radically from the psychoanalytic view that it led to his break with Freud.)
So long as the child is in that state of unconscious identity with the mother, he is still one with the animal psyche and is just as unconscious as it. The development of consciousness inevitably leads not only to separation from the mother, but to separation from the parents and the whole family circle and thus to a relative degree of detachment from the unconscious and the world of instinct. Yet the longing for this lost world continues and, when difficult adaptations are demanded, is forever tempting one to make evasions and retreats, to regress to the infantile past, which then starts throwing up the incestuous symbolism. ["Symbols of the Mother and of Rebirth," CW 5, par. 351.]
Whenever [the] drive for wholeness appears, it begins by disguising itself under the symbolism of incest, for, unless he seeks it in himself, a man’s nearest feminine counterpart is to be found in his mother, sister, or daughter. ["The Psychology of the Transference," CW 16, par. 471.]