Why does the Bible call Jesus the Son of David?
According to the flesh (not according to His divine nature) the Lord Jesus Christ, as man, was a descendant of King David of Israel. Read the accounts about the ancestors of Jesus in Matthew 1:1-17 and Luke 3:23-38.
But please remember that Jesus was no ordinary man, He was and is God, the Second Person in the Trinity, who became man (John 3:16) in
order to be able to die for our sins and to save us (1 Corinthians 15:3).
We must open our hearts when He knocks on the "door" if we really want to be His children (read Revelation 3:20).