Why? Because Jesus said so. Because the gospel plainly says so. That should be reason enough.
For reasons including that Jesus says that he was with God before the world began, that John the gospel writer says that Jesus was with God and was God before He Himself created the earth, Jesus plainly states that he was there when Satan was cast down to earth, and the LORD of the Old Testament identifies Himself as Jesus when he speaks.
Zechariah 12:10 KJV
(10) And I will pour upon the house of David, and upon the inhabitants of Jerusalem, the spirit of grace and of supplications: and they shall look upon me whom they have pierced, and they shall mourn for him, as one mourneth for his only son, and shall be in bitterness for him, as one that is in bitterness for his firstborn.
That isn't the prophet Zechariah who is "me whom they have pierced." That's the LORD speaking in that passage. And again,
Zechariah 11:12-13 KJV
(12) And I said unto them, If ye think good, give me my price; and if not, forbear. So they weighed for my price thirty pieces of silver.
(13) And the LORD said unto me, Cast it unto the potter: a goodly price that I was prised at of them. And I took the thirty pieces of silver, and cast them to the potter in the house of the LORD.
Again, the LORD is speaking when he says that HE is being priced at thirty pieces of silver. Who was this? Who is the "him" in Matthew 26:15?
Matthew 26:15 KJV
(15) And said unto them, What will ye give me, and I will deliver him unto you? And they covenanted with him for thirty pieces of silver.
Matthew 27:3 KJV
(3) Then Judas, which had betrayed him, when he saw that he was condemned, repented himself, and brought again the thirty pieces of silver to the chief priests and elders,
Matthew 27:9-10 KJV
(9) Then was fulfilled that which was spoken by Jeremy the prophet, saying, And they took the thirty pieces of silver, the price of him that was valued, whom they of the children of Israel did value;
(10) And gave them for the potter's field, as the Lord appointed me.
You may add those passages to what I have already provided, but that certainly isn't the last. If you faithfully reference Christ's sayings and the New Testament writings with the Old Testament he is revealed as the LORD and Creator, the same one and only God that created the heavens and the earth. Which also explains how Jesus can forgive your sins, why he has life in himself, why the grave could not hold him, why he can raise the dead, how come he said that he would raise himself.
John 2:19-22 KJV
(19) Jesus answered and said unto them, Destroy this temple, and in three days I will raise it up.
(20) Then said the Jews, Forty and six years was this temple in building, and wilt thou rear it up in three days?
(21) But he spake of the temple of his body.
(22) When therefore he was risen from the dead, his disciples remembered that he had said this unto them; and they believed the scripture, and the word which Jesus had said.
Not "and in three days someone else will raise me up" but "and in three days I will raise it up."
It also explains why the angels of God worship him, why we are to worship him, why we are to honor the Son on earth as we would honor the Father in heaven. If Jesus were not God, this would be blasphemy and rebellion like when Absalom tried to take the kingdom from David, or when Lucifer tried to take heaven from God.