Confronted with Mary's pregnancy, Joseph must, humanly speaking, have felt a certain sense of loss, for he did not yet know of the Annunciation. When the angel revealed the truth to him in his dream, Joseph had to learn, like Abraham to "hope against hope".
Joseph and Mary were engaged: their hope must have been for a happy married life, blessed with God's gift of children. Suddenly their whole world was turned upside down: God invited them to walk a different road. It required courage and hope to accept God's mysterious call. Human hopes and expectations were no longer sufficient to sustain them: they could only respond to the divine command "Do not be afraid" and put their trust and hope in God. Both Joseph and Mary received the same message, for they were both involved in this mystery.
We may think of Joseph almost as a bystander, while Mary was totally involved in the mystery of the Incarnation, but Scripture indicates otherwise: Mary and Joseph together had to "hope against hope". We can best understand Joseph's vocation through his association with Mary, just as we understand more of the mystery of Mary by looking to Joseph.
Father in Heaven, strengthen our hope so that we may respond to all your invitations, even when we cannot see what lies ahead.