Thorns in the Bible are symbols of aridity, lifelessness and pain. St Paul suffered from his thorn in the flesh which God wouldn't heal, but rather allow him to receive an abundance of Grace instead through that pain. Our Lord's head is surrounded by this dead, dried up wreath, scratching and puncturing His Holy scalp drinking deeply of the precious blood.
We too, being dried up dead things without Him, find ourselves revived by His blood which pours from His wounds which we by our unyielding aridity have caused. By His blood are we softened; our hearts become alive again with sap from the True Vine. By this mockery of His kingship, the thorns render that mockery meaningless, for the Blood of Christ redeems the world and thus reinforces the nature of His Kingdom within us.
I adore thee, O Lord Jesu Christ! hanging upon the Cross, and wearing on thy head the crown of thorns: I beseech thee that thy Cross may deliver me from the destroying angel. Amen