Why is Jesus called the son of David?

In the first verse of the first gospel as it appears in our New Testament, Matthew calls Jesus Christ “the son of David” as if it were a sort of preface to the genealogy he is about to write, and perhaps, a preface to Matthew’s entire testimony of the Savior.

David was the great king of ancient Israel. Christ is called the Only Begotten of the Father in the flesh; He had a mortal mother and an immortal Father. Therefore, in order for Jesus to be a literal descendant (and therefore a son) of David, Mary must have been of Davidic descent. Because David was king of Israel, his descendents, under the patriarchal order, were entitled to the throne. It is also significant that Jesus was born in Bethlehem, the City of David. In this sense Jesus shared a “geographical sonship” with the great King David.

Perhaps the analysis can go one step further if we distinguish “son of David” with a lowercase ‘s’ from “Son of David” with a capital ‘S.’ The lowercase ‘s’ version denotes any son or descendant of David. Absalom, Amnon, Solomon, and others are given this title (Cf. 2 Samuel 13:1, 2 Chronicles 1:1). But the title “Son of David” with a capital ‘S’ is reserved for only one being that has ever walked the earth. It is applied only to Jesus of Nazareth, usually when asked to perform a miracle. Hence, by addressing “Jesus as Son of David” one “demonstrates . . . belief that He was the Messiah of Israel” (Ibid. 335).
In Old Testament times, kings were anointed to their offices, as were priests, high priests, and prophets (Cf. 1 Samuel 10:1, Exodus 40:15, Leviticus 21:10, 1 Kings 19:16). It is, therefore, fitting that Jesus is called Messiah—the Anointed One—for He is Prophet, Priest, and King. David was anointed when he became king of Israel. David delivered Israel from political and religious oppression. It is therefore also fitting that the Anointed One—the Messiah—references the Heavenly King and the one who would ultimately deliver all people from sin and death. The Jews were “eagerly expecting” that Deliverer because the Old Testament is full of references, types, shadows, and prophecies of the coming of One who would unite Israel once and for all, give them the law of God to live by, and deliver unto them their nation. This Deliverer was and is Jesus Christ, the Son of David.