In Phantom Tollbooth why does humbug come along on Milo's journey?

Being that Humbug is the complete opposite of Milo's pal, Tock, he teaches Milo many different lessons. Humbug is selfish and arrogant whereas Tock is loyal and brave. Milo learns many lessons on humility and comes to realize through witnessing Humbug's misfortunes due to his attitude, that it will do him no good to be a selfish and arrogant person---it only leads to trouble.