Let's recall just how bad a jerk this younger brother is: first off, he comes up to his father, whom he should honour above all earthly things, and asks for his inheritance. That is, he is essentially saying to his father, "It'd be better for me if you were dead. Can you pretend to be dead so I can get what I want from you?"
The positive response of his father is in itself, scandalous to Jesus's audience. The younger son then distances himself from his father so as to completely control that wealth which is properly his fathers. Finally, after wasting his money on dissolute living ("sex, drugs and rock'n roll" to my students) he finds himself completely isolated and starving.
At this point in the story, I imagine the Pharisees and the scribes leaning back, perhaps smoking on their pipes if they have them, and muttering to each other, "That was a good story. I thought the father was a little bit of a putz, but at least the younger son got what was coming to him. Good story."
But Jesus isn't done yet. He continues with the younger son returning to a powerfully emotional welcome from his father, an acknowledgement on his lips: "I have behaved like no son of yours."
The father restores his sonship to the highest honour: sandals, the best cloak, a signet ring. The younger son still smells of pig poop. One only "kills the fatted calf" once in a lifetime.