What seems at first glance like three distinct episodes -- Jesus' triumphal entry, his weeping over Jerusalem, and the cleansing of the temple -- are in fact closely related and provide key insights into Jesus' identity and mission.
Palm Sunday sermons frequently point out the kingly symbolism in Jesus' triumphal entry, and rightly so. Rarely, however, do we take note of what Luke is at pains to point out: Jesus is returning just like the ruler in the story Jesus just told. Luke tells us that the triumphal entry happens "when he had said these things" -- a clear arrow pointing back to that story. So Jesus is entering Jerusalem to take up his kingship, and he will be no more welcome than the ruler in the story. His followers recognize what Jesus is doing, at least in part, and they hail him as king.
As Jesus comes down the hill -- the same hillside where he will be arrested in a few days -- he looks across the Kidron Valley to the city of Jerusalem, and he weeps for this city that he loves. He looks ahead to the day forty years in the future when the Roman legions will tear Jerusalem's walls to the ground and burn its temple. He states clearly that the things that could make for peace are hidden from the eyes of his people. Thematically, it's not hard to summarize what Jesus is talking about here: repentance (see Luke 13) and recognizing him as God's chosen king are the main things required for Jesus' people to know peace. What will occur in the next few days -- Jesus' rejection, betrayal, arrest, and crucifixion -- fly in the face of Jesus' prescription.
Perhaps it's Jesus' rejection is understandable, given what he does next in his presumptive authority as king: He enters the temple and cleanses it, driving out those who own the temple franchise, who change money for temple coinage and sell authorized sacrifices to worshipers. In spite of this in-your-face action, Jesus continues to teach in the temple daily, almost daring the authorities to silence him. They take their plotting underground, and the stage is set for Jesus to be betrayed and arrested. However, first we get to hear some of the exchanges that happen between him and the temple authorities during this tumultuous week.