8 The matter then being in this state, and all at their wits' end, the angel comes to solve all their difficulties. But it is worth inquiring, why the angel did not speak sooner, before the husband had such thoughts: but, “when he thought on it,” not until then, he came; for it is said, “While he thought on these things, the angel” comes. And yet to her he declares the good tidings even before she conceived. And this again contains another difficulty; for even though the angel had not spoken, wherefore was the Virgin silent, who had been informed by the angel; and why, when she saw her betrothed husband in trouble, did she not put an end to his perplexity?
Wherefore then did not the angel speak before Joseph became troubled. For we must needs explain the former difficulty first. For what reason then did he not speak? Lest Joseph should be unbelieving, and the same happen to him as to Zacharias. For when the thing was visible, belief was thenceforth easy; but when it had not yet a beginning, it was not equally easy to receive his saying. For this reason the angel spoke not at the first, and through the same cause the Virgin too held her peace. For she did not think to obtain credit with her betrothed husband, in declaring to him a thing unheard of, but rather that she should provoke him the more, as though she were cloking a sin that had been committed. Since if she herself, who was to receive so great a favor, is affected somewhat after the manner of man, and says, “How shall this be, seeing I know not a man?” much more would he have doubted; and especially when hearing it from the woman who was under suspicion. Wherefore the Virgin says nothing to him, but the angel, the time demanding it, presents himself to him.
Source: Homilies on the Gospel of St. Matthew (New Advent)