Right before his crucifixion, Jesus was made to carry his cross and to wear a crown of thorns. Why was he made to wear the crown of thorns?
He was being mocked for claiming that he was King of the Jews, so the soliders gave him a "crown." And to make it worse, they made it of thorns, so it was extremely painful.
As Ray asserted, the crown was mockingly placed on Him because Jesus was charged with the crime of being "the King of the Jews", while the Jews already had a king, at least according to the Romans.
There is, however, an important symbolism that is also present in this.
In Genesis 3, thorns and thistles are part of the curse of the first sin.
The prophet Isaiah specifically predicted that God would lay upon the Messiah the sin of the world.
Thus, as the soldier placed a crown of thorns on the head of Jesus, this actually symbolized the spiritual reality that God was placing upon Jesus the curse of sin.