Two groups of people heard Jesus telling this story; there were the bad guys—the sinners and tax collectors and there were the ‘so called’ good guys—the Pharisees.
One group was so bad they didn't think God would ever accept them. The other was so good they thought God had already accepted them. Both groups were wrong.
We always tend to focus on the son who comes back home, but Jesus’s focus is on the father.
The father and his relationships are the central teaching of this parable.