Certain General Principles of Parabolic Interpretation. These Applied to the Parables Now Under Consideration, Especially to that of the Prodigal Son
But it is more (to the point) that it is not lawful to draw conclusions about anything else than the subject which was immediately in hand. In short, if it were lawful to transfer the parables to other ends (than they were originally intended for), it would be rather to martyrdom that we would direct the hope drawn from those now in question; for that is the only thing which, after all his substance has been squandered, will be able to restore the son; and will joyfully proclaim that the drachma has been found, albeit among all (rubbish) on a dungheap; and will carry back into the flock on the shoulders of the Lord Himself the ewe, fugitive though she have been over all that is rough and rugged. But we prefer, if it must be so, to be less wise in the Scriptures, than to be wise against them. We are as much bound to keep the sense of the Lord as His precept. Transgression in interpretation is not lighter than in conversation.
Source: On Modesty (New Advent)