Understanding the Prodigal Son parable requires understanding why Jesus told the parable in the first place. He was speaking to a very mixed crowd. In Luke 15:1-2 there are at least 4 groups, sinners, tax collectors, Scribes and Pharisees. Jesus tells this series of three parables to drive home a simple point: God loves the lost and His heart's desire is to find all those separated from Him. Each parable gives a little different spin on His point.
In the third parable, Jesus gives two parts to the parable. In the first part, the younger son is the object of the parable. In the second half, the elder brother is the focus. Each of these parts speaks to the two classes of people to which Jesus was speaking i.e. the sinners and the Pharisees. The first would speak to those who had been separated from God (were prodigal in their relationship with God). The second half would be directed toward the Pharisees and Scribes (those who hadn't departed from God physically).
As I study this parable, it becomes apparent to me that neither son understood the father. The younger son thought that doing righteous things makes you worthy of the father's love. Wrong!
The older brother thought the same thing. Instead, the father was not focusing upon the behaviors of either son. He wanted to have a relationship of love with both brothers. He demonstrates this when he doesn't listen to the smooth lines the younger brother has rehearsed. He also says it to the older brother when he says, " Child, (a different term in the Greek than the word for son, "hwion") you are always with me, and all my things are yours. But it was necessary to kill the fatted calf and celebrate because your brother was dead and is now alive, he was lost and now he is found.
The father's heart desired whole relationships with both his sons. Neither understood. Could it be that in our attempt to be worthy of God's love, we miss what God is really trying to say to us: that we have a priceless value to Him, clearly demonstrated upon the cross.