Where thorns come from
Before the fall, there are no thorns in the Bible. They first appear the moment Adam sins, as part of his sentence:
"Cursed is the earth in thy work: with labour and toil shalt thou eat thereof all the days of thy life. Thorns and thistles shall it bring forth to thee."
Genesis 3:17 to 18
Thorns are what the curse looks like. Every thorn in the ground is a reminder of the first sin.
The crown
Now jump to the morning Jesus was condemned:
"And stripping him, they put a scarlet cloak about him. And platting a crown of thorns, they put it upon his head."
Matthew 27:28 to 29
The soldiers wanted to mock a king. Of everything they could have used, they reached for thorns. They had no idea what they were holding: the mark of the curse, pressed onto the head of the one who came to undo it.
He became the curse
Paul says this outright:
"Christ hath redeemed us from the curse of the law, being made a curse for us."
Galatians 3:13
He didn't just suffer under the curse. He took it on himself. And the thorns on his head show it.
Two Adams
Paul calls Jesus "the last Adam":
"For by a man came death, and by a man the resurrection of the dead. And as in Adam all die, so also in Christ all shall be made alive."
1 Corinthians 15:21 to 22
The first Adam brought the curse into the world, and the ground answered with thorns. The last Adam took those same thorns on his own head and carried the curse out.
The sign of a king
A crown is a sign of rule. Adam was given rule over creation, and lost it. The thorns are the sign of what he lost.
Jesus takes the sign of that loss and wears it as a crown. The emblem of defeat becomes the crown of the true King.
And the sweat
Adam's sentence had a second half: "In the sweat of thy face shalt thou eat bread" (Genesis 3:19).
On the night before he died, in the garden of Gethsemane, Jesus prayed in agony, "And his sweat became as drops of blood, trickling down upon the ground" (Luke 22:44).
The sweat. The thorns. Both parts of Adam's curse, laid on Jesus within a few hours.
Why this is a big deal
Thorns appear at the very start of the Bible as the mark of the curse. They appear again at the end, on the head of Christ. Same object, both ends of the story.
He wore the curse, and wore it out of the world.