There are many good reasons why St. Joseph should be the special heavenly patron of dedicated souls - in the religious life, in the priesthood, and among the laity. But as the Church teaches, he is especially to be venerated and his patronage invoked because he was the guardian of the Virgin Mary and the foster-father of Jesus. The dignity of the Mother of God is so sublime that nothing created can rank above it.
But as Joseph was united to the Blessed Virgin by the ties of marriage, we may believe that he approached nearer than anyone else to the eminent dignity by which the Mother of God surpasses all human persons in holiness.
Marriage is the most intimate of all unions, which from its essence confers a sharing of gifts between those joined together in wedlock. Thus, in giving Joseph the Blessed Virgin as his spouse, God appointed him to be not only the witness of her virginity, the protector of her honor, but also, because they were truly married, a sharer in her exalted sanctity.
Link (here) to read the full piece on Saint Joseph by Father John Hardon, S.J.