Jesus is starting to attract unsavoury types like tax collectors and other, undefined 'sinners.' The Pharisees don't like this, but it's hard to imagine them liking anything. Jesus' retort is perhaps the first trolley problem: if they had a hundred sheep, and one wandered off, wouldn't they leave the other 99 alone in the wilderness while they looked for the lost one, and then throw a party when they found one? I dunno. Frankly that seems like an awful lot of effort for one sheep and pretty risky to leave 99 of them alone in the wilderness, but I'm no shepherd.
He tells another parable that I can identify with much more: a woman who has 10 pieces of silver, loses one, and turns the house upside down looking for it, then throws a party when she finds it. I hope the party costs less than a piece of silver. Anyway, Jesus says he feels the same way when he convinces a sinner to repent.
Now he has a sad parable for us: the younger of two brothers asks his father for his inheritance, which he then spends partying it up in another country. As in all cautionary tales, things quickly go wrong: there's a famine and the young man falls on hard times. So hard that he is forced to take up a position as a swineherd. In a rather illustrative argument for the welfare state, the man is so poorly paid and fed that the pigs' feed starts to look good to him. He finds himself thinking about how his father has lots of hired servants, all of whom eat better than him. He resolves to go home and throw himself on his father's mercy, and to become a servant if need be. However, he gets lucky, and when his father sees him, he hugs him and calls for the boy to be dressed in fine clothes and shoes, and for a feast. The boy tries to protest, but is ignored.
Now, his dutiful older brother has been out in the field this entire time, and didn't even realise there was a party going on until he hears the music on his way back from a hard day's work. He asks one of the servants what's going on, then refuses to enter the house in disgust. His father comes out to entreat him to join, and then the long-stored resentment comes out: he's been working his ass off for years and his father never so much as gave him a goat kid to roast for a party! And now this buffoon, who spent his entire inheritance on whores, gets a whole fatted calf! It's not fair! The father assures him that he'll get his full inheritance, but they had to have a party, see, because the prodigal son was dead and is now alive. We'll see what's left when the will is read.