That Jesus was born of a virgin is one of the most marvelous aspects of the miracle of the Incarnation. While not even considering objections from skeptics here, it is not uncommon to hear Christians raise the question, "Why was Jesus born of the Virgin Mary?" Answers typically revolve around the need to fulfill prophecy (cf. Isaiah 7), show God's providential initiative, or avoid the transmission of sin*.
Reading through Justin Martyr's Dialogue With Trypho (according to the Read the Fathers reading plan), St. Justin presents the following reason for Jesus' virgin birth:
He became man by the Virgin, in order that the disobedience which proceeded from the serpent might receive its destruction in the same manner in which it derived its origin. For Eve, who was a virgin and undefiled, having conceived the word of the serpent, brought forth disobedience and death. But the Virgin Mary received faith and joy, when the angel Gabriel announced the good tidings to her that the Spirit of the Lord would come upon her, and the power of the Highest would overshadow her: wherefore also the Holy Thing begotten of her is the Son of God (Dialogue With Trypho, Chapter 100).
In other words, since sin entered the world through a virgin, Eve, Christ was born of the virgin Mary that sin might also be destroyed through a virgin.
His is an interesting one, to say the very least.
* This view only makes sense, of course, if sin is transmitted by male DNA or you further postulate Mary's Immaculate Conception...neither of which is supported from Scripture or Church Tradition outside of Roman Catholicism.