Beautifully expressed by the pope.

The father's joy doesn't derive from any desperate, practical expedience on the part of the son but from his loving act of reconciliation.

As Balthasar says, the father sees the son as he really is, having elected him in Christ before the foundation of the world. And once again and for now, at least, the son is acting like one.

The story gives me some light on what it means to be co-heir by participation in Christ.

Christ's love was complete in the utter desolation and abandonment of Calvary, freely undertaken.
Ours, while subject to sinfulness and dependant on grace, can only be commensurate after having similarly passed through the crucible of suffering and abandonment, even if self-imposed.

Suffering and joy are two ends of the same stick.