“An angel of the Lord appeared to him in a dream and said, ‘Joseph, son of David, do not be afraid to take Mary home with you to be your wife. It is through the Holy Spirit that she has conceived.”
(Matthew 1:20)

I guess the angel that is now appearing to Joseph is no other than Gabriel. He has set in motion the process of the Incarnation with Zechariah and Mary, and he brings it now to a close with Joseph. He is thorough in his work and does not do things by half. I also guess that Gabriel was worried because he knew the difficult moment Mary was passing through, and felt a little responsible for her perplexity. To Mary and Elisabeth he had properly explained the virgin birth, which with them was a matter of unmixed joy and wonder as their faith was backed by the miracle, first announced and then fulfilled, of Elizabeth’s pregnancy. But Joseph knew nothing about it, while it all concerned him very closely. He was mystified before the tense situation, anguished by social pressure and inner doubt. What should he do?

The angel has waited, because that was God’s order, and he knows well that God sometimes tests those he loves to heighten their virtue and to increase their faith. But as soon as he is allowed, the angel flies to Joseph, explains the situation to him with his usual clarity and brevity, and can now go back to his kingdom with the satisfaction of a well accomplished mission. Joseph takes Mary home, and a child is born with the name the angel had given him: Jesus.