Christ is using this parable to teach the Pharisees, and us, about hypocrisy and about God’s love.
Here is the hard lesson: The Pharisees have a problem, and it is one that we still have to contend with. Their difficulty was that their “inner man” did not match the “outer man.” We, like they did, walk around with the appearance of holiness and righteousness, but our hearts are hard and unforgiving. We can hide our hearts from others, for a while.
This behavior and self-deception, this hypocrisy, has nothing to do with the love and grace of God. It has nothing to do with the love and grace we should extend to ourselves and to others. Nothing!
There's a another reason this short story pulls at our heartstrings. We recognize ourselves in it. We are in it! The parable reminds us of the most painful aspects of the human condition, and those who are willing to take an honest look will recognize themselves.
For believers, the Prodigal Son is a humbling reminder of who we are and how much we owe to divine grace.
For sinners coming to repentance, the father's eager welcome and costly (to Him) generosity are reminders that God's grace and goodness are inexhaustible.
For firm unbelievers, especially those like the scribes and Pharisees, who continue to use their external display of righteousness as a mask for their unrighteous and unyielding hearts, the elder brother is a reminder that neither a show of religion nor the pretense of respectability will save us.