"And when they had plaited a crown of thorns, they put it on his head and a reed in his right hand...and they spit upon him and took the reed and smote him on the head" (Matthew 27:29-30).
Why a crown of thorns? I can understand the scarlet robe, the hyssop. They were used in the Red Heifer Sacrifice (Numbers 19). Jesus said the He did not come to destroy the law and prophets but to fulfill them. He said not one jot or tittle would pass away until all was fulfilled. He fulfilled completely the Red Heifer Sacrifice. But, why the crown of thorns?
The book of Genesis gives us the answer. "...cursed is the ground for thy sake...thorns and thistles shall it bring forth to thee..." (Genesis 3:17-18). Jesus wore a crown of thorns to take the curse from the ground. He bore the curse for us.