The part of the Nativity that has really struck me this year is God's omniscience - that God, being omniscient, cannot be surprised. Anything that happens to Him, happens because He has decided that that is what will happen. As Jesus said about His life, "No one takes it from me, but I lay it down of my own accord."
So when we come to the Nativity, we come to a story in which everything that happened was chosen by God. Jesus was the only person ever to be born who was able to choose the circumstances of their birth - the family they would be born into, their social class, their country... and in Jesus, we see someone who chose the opposite of what most, if not all, people would choose.
Jesus was not born into a wealthy family, part of the elite of society. He wasn't born in an important city. His birth was not anywhere of note. Instead, Jesus chose to be born to a poor couple, in a little town of little significance in the worlds eyes, a town that couldn't even supply Him with a room to be born in, to a people and country oppressed by a brutal foreign occupation. The person who could have had Kings attending His birth, instead chose shepherds, the people near the bottom of the social pyramid. He could have had the company of the rulers of men at His birth; instead, He was born among animals.
The story of Christmas, is the story of God choosing to identify with the poor, the oppressed, and the lowly. He chose to say of those who the world looks down on, "These are my people, and this is my family."