The context is the jealously of the self-righteous, outwardly pious Scribes and Pharisees (Matthew 23:28) on account of the Lord's eating with sinners:
Luke 15:1-32 (DRB) Now the publicans and sinners drew near unto him to hear him. 2 And the Pharisees and the scribes murmured, saying: This man receiveth sinners, and eateth with them.
To eat with sinners in public was more than a private matter, but to promote and endorse, by implication, their way of life. After all, in their mind, why else would one put up with these people if they were 'true Jews?' Hence the scandal of this episode.
And he spoke to them this parable, saying: 4 What man of you that hath an hundred sheep: and if he shall lose one of them, doth he not leave the ninety-nine in the desert, and go after that which was lost, until he find it? 5 And when he hath found it, lay it upon his shoulders, rejoicing: 6 And coming home, call together his friends and neighbours, saying to them: Rejoice with me, because I have found my sheep that was lost? 7 I say to you, that even so there shall be joy in heaven upon one sinner that [repeneth], more than upon ninety-nine just who need not [repentance].
What 'good shepherd,' that is, would not do this for the sake of his lost sheep? Only a bad shepherd allows his sheep to stray, without seeking them:' You, Pharisees, are bad shepherd of the people, and these are your potential sheep!'
"I am the Good Shepherd," Christ says (John 10:11).
8 Or what woman having ten groats; if she lose one groat, doth not light a candle, and sweep the house, and seek diligently until she find it? 9 And when she hath found it, call together her friends and neighbours, saying: Rejoice with me, because I have found the groat which I had lost. 10 So I say to you, there shall be joy before the angels of God upon one sinner [who repents].
Thus, the sinners are the thing sought in both cases, both received with joy, not shunned and cut off from all hope of restoration to God, as the Pharisees were doing in practice.
And he said: A certain man had two sons: 12 And the younger of them said to his father: Father, give me the portion of substance that falleth to me. And he divided unto them his substance. 13 And not many days after, the younger son, gathering all together, went abroad into a far country: and there wasted his substance, living riotously.
As we see further in the parable, these 'sons' stand for the Jews as the older, the firstborn, and the other peoples, the Nations, the Gentiles as a whole, who, since they 'come along after,' 'dragging at the heel' (cf. Genesis 25:26) of the Jews, are the younger brother. The Gentiles being spread across the world, 'a far country.' Living riotously represents the idolatry and worldliness which characterize the Gentiles of virtually all times and places, squandering their former sonship of God by spiritual alignment with the devil.
And after he had spent all, there came a mighty famine in that country; and he began to be in want. 15 And he went and cleaved to one of the citizens of that country. And he sent him into his farm to feed swine. 16 And he would fain have filled his belly with the husks the swine did eat; and no man gave unto him. 17 And returning to himself, he said: How many hired servants in my father's house abound with bread, and I here perish with hunger? 18 I will arise, and will go to my father, and say to him: Father, I have sinned against heaven, and before thee: 19 I am not worthy to be called thy son: make me as one of thy hired servants.
Like Ruth, "when ..there came a famine in the land," "arose to go from the land of Moab to her own country with both her daughters in law," this son "had heard that the Lord had looked upon his people, and had given them food." (Ruth 1). That is, spiritual restoration to the original inheritance state, the spiritual abundance of food from the famine of spiritual death, idols, fornication etc. A yearning for home, inspired by the good things God offers to His children.
And rising up he came to his father. And when he was yet a great way off, his father saw him, and was moved with compassion, and running to him fell upon his neck, and kissed him. 21 And the son said to him: Father, I have sinned against heaven, and before thee, I am not now worthy to be called thy son. 22 And the father said to his servants: Bring forth quickly the first robe, and put it on him, and put a ring on his hand, and shoes on his feet: 23 And bring hither the fatted calf, and kill it, and let us eat and make merry: 24 Because this my son was dead, and is come to life again: was lost, and is found. And they began to be merry.
Those who were outside the fold, the younger, latecomer son is occasion for a great celebration because he was meant to be home all along as at the start in Adam ("lost" is used of something displaced from its proper place). Not only is he accepted, but such is the degree of joy at the repentance of this son that is cause for hyper-reaction, an overflow of graces.
Now his elder son was in the field, and when he came and drew nigh to the house, he heard music and dancing: 26 And he called one of the servants, and asked what these things meant. 27 And he said to him: Thy brother is come, and thy father hath killed the fatted calf, because he hath received him safe. 28 And he was angry, and would not go in. His father therefore coming out began to entreat him.
Here is where Jesus starts to leave off the parable, and in doing so it brings them round in a circle to before He answered them with the words, "What man of you that hath a hundred sheep..." They are the older son who sees so many unearned blessings bestowed on the younger brother. They are the chosen fold. But since "other sheep I have that are not of this fold" (John 10:11), and He welcomes them into the fold, they are "angry" and quite obviously jealous.
And he answering, said to his father: Behold, for so many years do I serve thee, and I have never transgressed thy commandment, and yet thou hast never given me a kid to make merry with my friends: 30 But as soon as this thy son is come, who hath devoured his substance with harlots, thou hast killed for him the fatted calf.
Can't you just hear it?
Luke 18:11-12 (DRB) The Pharisee standing, prayed thus with himself: O God, I give thee thanks that I am not as the rest of men, extortioners, unjust, adulterers, as also is this publican. 12 I fast twice in a week: I give tithes of all that I possess.
But he said to him: Son, thou art always with me, and all I have is thine. 32 But it was fit that we should make merry and be glad, for this thy brother was dead and is come to life again; he was lost, and is found.
How does Jesus leave off with this parable? By leaving it up to the Pharisees he is currently speaking with: 'What will you do now that it's clear that I am the Good 'Shepherd,' these are my lost sheep which I have sought and found, and am now making merry with? And you are the enemies 'in whose presence I set a table for them' (Psalm 23). Will you question the Good Shepherd, you who are evil shepherds who devour the flock?'
Romans 9:30-33 (DRB) What then shall we say? That the Gentiles, who followed not after justice, have attained to justice, even the justice that is of faith [Luke 15:18-19]. 31 But Israel, by following after the law of justice, is not come unto the law of justice. 32 Why so? Because they sought it not by faith, but as it were of works [Luke 15:29-30]. For they stumbled at the stumblingstone. 33 As it is written: Behold I lay in Sion a stumblingstone and a rock of scandal; and whosoever believeth in him shall not be confounded.