The sight of Jesus riding a donkey was a Messianic prophecy fulfilled.  Zechariah 9:9 says, "Rejoice greatly, Daughter Zion! Shout, Daughter Jerusalem! See, your king comes to you, righteous and victorious, lowly and riding on a donkey, on a colt, the foal of a donkey ..."  The crowd understood that prophecy was being fulfilled in front of their eyes and they shouted their praise.  

Not so the Pharisees.  They mingled with the crowd, spying on Jesus and trying to catch Him breaking their law or saying something that would incriminate Himself.  Usually they kept quiet but, keen to avoid the religious embarrassment of having crowds openly confessing Jesus as the Christ, they rebuked Jesus and told Him to rebuke the crowd.
 
Jesus' response was swift, "If they keep quiet, the stones will cry out!"  The crowd were not being deceived.  They were telling the truth.  Jesus really was Father God's chosen messenger.  And so, with memories of Jesus' ministry including the miracles, everything came together in their minds as they confessed Jesus as the Christ (the Anointed One = Messiah).  So they added other Old Testament Scriptures to the chanting. Psalm 118:25-26 says, "O Lord, save us; O Lord, grant us success. Blessed is he who comes in the name of the Lord. From the house of the Lord we bless you." This was not a manipulated crowd, and an attempt to stir up popularity.  No, it was a spontaneous public demonstration of how they regarded the Lord Jesus Christ.
 
It is right to praise God, and for the crowd to audibly honour the Saviour, in the same way it is right for us to join in praise and worship.  But, what about this spontaneous outburst ... should that ever be our experience?  Yes! (Psalm 109:30)  The Psalms are full of such expressions, sparked by the beauty of His creation, the memory of His wonderful deeds, the largeness of His mercy to sinners, the wonder of His intimate knowledge and the completeness of His salvation.  So, when the Lord reveals His character to you, praise Him!  When prayers are answered, and whenever we realise that the Lord has been involved in our lives, spoken praise is natural.  Indeed, to refrain from praise is disobedience.  Don't wait for a church meeting until you praise; praise Him and ask two or three others to join you - praise Him whenever you can!