Zechariah is calling the "daughter of Zion" the "daughter of Jerusalem" to do two things.  To "rejoice greatly" and to "shout aloud."  Notice that Zechariah is not calling for simple rejoicing or common shouting, but he intensifies them with the modifiers "greatly" and "aloud" to draw our attention to his call.  We feel led to inquire why these modifiers?  What would cause this great rejoicing and this loud shouting? 

Zechariah answers with an emphatic "Behold!" which only focuses our attention further on the answer soon to be given.  The reason is that the King of the "daughter of Zion" and the "daughter of Jerusalem" is coming.  Yet, he is not only coming, he is coming to them!  We should then ask, why is this exciting?  I'm not fully convinced this is enough grounds to call for great rejoicing and shouting loudly. 

Zechariah continues to give evidence as to why the people should respond so extravagantly to this word by stating that this king of theirs that is coming to them is righteous and he has salvation!  This king is a perfectly just king!  He is not corrupt!  He is righteous!  What a wonderful thing it is to live in the kingdom ruled by a righteous king, Zechariah seems to be saying. So rejoice!  He also has salvation.  He is not one who may deliver, but he will deliver!  He himself seems to be the essence of the deliverance!  That salvation for the people is contingent upon him coming to them! 

If this isn't enough reason to rejoice, Zechariah continues to explain who this king is by saying that he is a humble king.  Having a character defined by humility is an uncommon pattern in the list of kings who ruled over Israel.  Instead of amassing horses for himself to display his own might, this king appears on a donkey.  Not only a donkey, but even more fragile, the foal of a donkey.  This king seems to have a confidence that lies outside of himself or his possessions.  This is a king of faith.  This king is a picture of Christ the King, and we can rejoice that he HAS come to US his children!  May we rejoice greatly and shout aloud!