Some editions of the Bible add a title to this Gospel passage and speak of "Jesus' glorious entry into Jerusalem." From our perspective that may very well be a realistic evaluation of the significance of this event in the whole Passion and Resurrection account. St John, however, writes in such a way as to give us a deeper insight into Jesus' mind and heart.
While Matthew and Luke have detailed accounts of Jesus facing temptation in the desert, John puts these temptations back into his daily life, thus teaching us that our temptations usually come to us through the questions, doubts, aspersions and antipathies of those who have no faith and cannot understand ours.
Thus in the desert the devil tempted Jesus to seek the power of an earthly king and kingdom. In real life, several groups of people suggested or hinted to Jesus that he should be king.
Thus, according to St John, Jesus sensed the tendency of the crowd and his disciples and went himself to get the donkey as if to say: "If you desire me to be a king, remember the kind of king which the prophet spoke of: one lowly and humble and riding on a donkey, a king whose only power is the gift of peace."
Lord, grant us your peace and so be king of our hearts.