If Christmas is Jesus' birthday, then why are we the ones who get the presents?
There is a historical answer to that question: pagan cultures had a tradition of giving gifts in winter, and as people from those cultures came into Christian faith, their gift-giving was folded into their new Christian practice. But, while true, that is not really our answer to the question.
There is an answer from the nativity: the Magi brought gifts to baby Jesus and we want to do as they did. But that alone does not explain why we receive the gifts. Stephen Nissenbaum, author of "The Battle for Christmas," says that in early modern Europe, bands of poor, rowdy, young men would "wassail" from house to house among the gentry and demand handouts. In the 1800s Christmas became more and more of a family holiday and the giving of gifts moved from one class to another, and from one generation to another. But that is a cultural and sociological answer to the question and it, too, is not our answer.
My answer is a creative one—I mean that literally. I believe our answer comes from creation itself. In the time before time, when there was nothing else but God, something inside of God said something like, "Life is too good to hoard. I am going to make others who are like Me to share in My joy." That is to say, all that is came about because of the generosity of God. God created in order to give something of Himself, not to get something for Himself. Genesis 1 says God spoke the creation into being. "And God said, 'Let there be light,' and there was light." The Prologue in John's Gospel says the Word God spoke was Jesus. That is, at least in part, a poetic way of saying Jesus is the very essence of God's generosity. So if God created all that is to give and not to receive and if Jesus is the essence of His generosity, then what could be more appropriate on Christmas than receiving? After all, Jesus is God's first and best gift. So we receive gifts because God's deepest nature is to give and to bless and we want to embody that nature.
By the way, if it seems to you like all this receiving, without real and generous giving to the Kingdom of God, puts us out of alignment with the heart of God, I think you are right. Perhaps we ought to consider what we want to give this year. It is not too late. Perhaps we can live into the heart of God by living out the practice of God.