Why is Jesus called "the Son of God"?
The truth is that Jesus himself said that He was the Son of God: "Then they all said, 'Are You then the Son of God?' So He said to them, 'You rightly say that I am." (Lk. 22:70 NKJ).
"He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God" (Jn. 3:18 NKJ). Men also gave Him that title: She said to Him, "Yes, Lord, I believe that You are the Christ, the Son of God, who is to come into the world" (Jn. 11:27 NKJ).
Satan and demons called Him by that title too: When he saw Jesus, he cried out, fell down before Him, and with a loud voice said, "What have I to do with You, Jesus, Son of the Most High God? I beg You, do not torment me!" (Lk. 8:28 NKJ).
So why did God, man and demons give Him that title? In eternities past, the three persons of the Triune God (Godhead) took subordinate roles to each other. While they were equal to one another, they took on their respective roles in order to fulfill their redemptive plan for mankind. The First Person of the Godhead became the Father--Jehovah, Yahweh, The Great I AM.
The Second Person of the Godhead took on a subordinate role to the Father and first became the Word then the Word became flesh and is known as Jesus, King of Kings and Lord of Lords, Immanuel, Son of Man, Son of God, Christ, and perhaps Angel of the Lord. The Third Person of the Godhead took on a subordinate role to the Son and the Father and was known as The Spirit of the Lord, Holy Spirit, The Comforter. This is why Jesus is called "the Son of God".