My answer might be:
God gave man the perfect gift of life, one without pain or death. Man destroyed that bond when he marred it with the blackness of sin. Why would God want to give us anything when we destroyed that beautiful familial bond?
Yet He did. Because He loved us, He kept giving us chances. We were the ones corrupt with sin, murdering one another, destroying what He built. Sacrifices done in faith to God cleansed our sins. Yet the covenant was still broken and only one thing would repair it. In accepting what God asked her, Mary, in perfect obedience, began the process of our final repair to the broken relationship. Jesus, in his perfect sacrifice, finished it. Jesus, the sacrificial lamb, established a new covenant, one in which the gift of the Holy Spirit was given to unite us once again to God. Like blood ties, the Holy Spirit unites us like family. There are no more blood sacrifices after the perfect sacrifice, Jesus. (Note that none of his bones were ever broken, another requisite for a perfect sacrifice).
I think this is the best response to the question I’ve seen. I might add one other thought. Your friend’s comment about a bloodthirsty God also reveals a misunderstanding of the nature of the Trinity. Isn’t it fair to say that our God did not simply demand the death of his Son, but that in fact our God laid down His own life for our salvation, since it is true that we are talking about one God in three Persons, one of whom is the Son, Jesus Christ?
Tempus Fugit Memento Mori