According to the custom of the times, the betrothal of Joseph and Mary was arranged by their parents.
What is it that attracted Joseph to Mary?
"Mary virginal love radiated upon her entire body and physical life, infusing into them a luminous interior purity". (Thomas Philippe, OP) Joseph saw in Mary her purity of soul and fell in love with this beauty. We may very well apply to Mary the image of an enclosed garden.
"You are an enclosed garden,
my sister, my bride;
an enclosed garden,
a fountain sealed." (Sg 4:12)
Before the Betrothal, Joseph surely discussed with Mary her lineage, her descent from the the House of David. Mary, on her part, made known to Joseph her intent to remain a virgin. This was contrary to the culture of Israel which viewed a childless woman as a disgrace. Joseph, having learned to seek happiness in God and being aware of the mysterious ways of God, consented.
In the Jewish custom, Joseph prepared a Marriage contract (verbal or written) which he presented to the bride and her father before witnesses. In it was the "Mary's Price" to compensate the bride's parents for raising her as well to express his love for her. The proposal was accepted and Joseph poured a cup of wine and presented it to Mary who drank it. The cup of wine represented the Blood of the Covenant.
The Betrothal was concluded with benediction--
"Blessed art Thou, O Lord our God, King of the World,
Who sanctified us by his commandments,
and enjoined us about incest, and forbidden the betrothed,
but allowed us those wedded by Chuppah and betrothal."
Alfred Edersheim, Jesus the Messiah, Hendrickson Pub., Mass.,1993, pp. 105-105
This must have been a very happy day for Joseph and Mary.
Sometimes after the Betrothal, an angel of the Lord appeared to Mary at Nazareth and told her that she had been chosen to bear the Son of God. Mary, although troubled at first, said: "'Be it be done to me according to your word.'" Mary did not tell Joseph this event probably feeling that he would not have understood. Instead she went to visit her cousin Elizabeth who was in her sixth month of pregnancy.
4. Joseph's Sorrow and Enlightenment