Why did God make the Virgin Mary pregnant with his son?

I guess he was thinking ahead. It makes a good plot for a book and quite a few movies.

Why are all these son of god stories so similar?

This piece of fiction's plot line has been repeated over and over again in human mythology.

The question you ask is ill posed,

From what I remember it is because of King Davids bloodline that Mary was chosen.

Why?

Well, I've never asked Him specifically, but apparently He had it in mind very early on---Gen 3: 14-15

And the Lord God said to the serpent: Because thou hast done this thing, thou art cursed among all cattle, and the beasts of the earth: Upon thy breast shalt thou go, and earth shalt thou eat all the days of thy life.

I will put enmities between thee and the woman, and thy seed and her seed: She shall crush thy head, and thou shalt lie in wait for her heel.

And miscellaneous:  

O truly necessary sin of Adam,

Destroyed completely by the Death of Christ!

O happy fault that earned so great,

So glorious a Redeemer

[found in the Exsúltet (the Easter Proclamation)]

The Latin expression felix culpa (happy fault) is derived from the writings of St Augustine

A virgin is to supply Jesus’ human body, for such a birth was foretold. (Isaiah 7:14; Matthew 1:22, 23) But why is a engaged virgin required? In order to provide an adoptive father who can convey to the child the legal right to King David’s throne. Both Joseph and Mary belong to the tribe of Judah and are descendants of King David. So the heirship rights of Jesus will be doubly established. (Matthew 1:2-16; Luke 3:23-33) This is why the angel later assures Joseph that he should not hesitate to take Mary as his legal wife, although she is pregnant. (Matthew 1:19-25)

Because it was popular Lore of the time . . .

Hundreds of years prior to the Virgin Mary's story, there are other Man-Gods born of virgins.

When one states questions regarding the story of the virgin birth, the follow up answer should always be "Which One?" . . .

What some mystic texts will tell you :- The "house of bread" is the constellation Virgo, the virgin. Hence in Bethlehem, the House of Bread, you will find a virgin, the constellation Virgo. Virgo is always depicted as holding either a sheaf of wheat or an ear of corn. She represents harvest time. As the Sun moves through the Zodiac on it's yearly journey, it eventually passes out of the House of Bread and moves on to the next house. The Sun moves out of the constellation Virgo. Virgo, the eternal Virgin, figuratively gives birth to the Sun, as the Sun passes out of Virgo the Virgin.

Jesus in the bible may represent Pisces the fish. The house the sun had entered into. He told his followers to go to the man carrying a pitcher of water (Aquarius) and meet him there, which is the next age or aeon. The wisemen were astronomers.

Either this is the whole story of the bible, or the bible is telling more than one story and the story of the sun passing through the zodiac and being hung on the cross of the zodiac has been added to it.