The Megila begins, "and it was in the days of Achashveirosh"; why doesn't it refer to him as "King Achashveirosh?" The Menos Levi says that the Neviim could not give Achashveirosh this title of respect because he had canceled the building of the Bais Hamikdash, and with that negative decree he lost some of his kingly powers in the spiritual world. Others say that he wasn't called "king" because he himself was insecure in his position, and thus was never truly comfortable wearing that title.
This teaches us to know that one must earn his title; respect must be earned; it does not come merely by a "crown being worn on one's head." One must behave in a manner befitting a king by acting appropriately in the spiritual sense and at the same time recognizing one's role in this world.