...at the Annunciation, the choice that Mary makes in her fiat is something altogether different, because it is a choice of the gift that the Father gives her of His Son. This is a gift that completely surpasses her, and it is only in a wholly pure and contemplative faith that she is able to receive it. It is for this reason that this gift plunges her into silence, a silence which binds her to the will of the Father, to the Holy Spirit Who “came upon her” and to the One Who is given to her: her Son. It is impossible for her to communicate this secret.
So Mary keeps the secret of the Annunciation, and she can do so because she trusts Joseph. We share a secret with someone because we trust that person, but sometimes we need to keep silent, and it is the mark of an even greater trust. Mary could ask anything of Joseph, and she asked him to endure this silence. –Father Marie-Dominique Philippe ‘The Mystery of St Joseph’