Jesus makes his point clear by telling the story in three parts. First thing to note is who was the parable told for? The parable was for tax collectors, sinners, Pharisees and Scribes. In my opinion the message applies to all four groups, but it was mainly for the Pharisees and Scribes. In the Parable of the Lost Sheep it has a pattern of losing something(a sheep) and finding it then rejoice TOGETHER. In the Parable of the Lost Coin it follows the same pattern; something is lost(a coin) but is found then is followed by rejoicing TOGETHER. Similarly the prodigal son was lost then found but was there rejoice TOGETHER? As you may have notice i capped TOGETHER to bring in a point. The older son did not rejoice together with the family. This is how the Pharisees/Scribes were. Even though Jesus was bringing people to God(teaching to repent,etc) the Pharisees tried to stop him and plotted against him. It is similar in a way how Jesus use "Should he heal on the Sabbath?". Rather than being bitter about it, they should rejoice. If Jesus is bringing a new soul toward God and if we are all children of God, we should rejoice! If the Pharisees/Scribes are all children of God they should rejoice together when a sinner is admitted in.
Also notice how Jesus did not say how the older son replied, which is smart because it was a question for each Pharisee as an individual to answer.
It is also incorrect to say there is only one thing you should learn from this parable, but many things. As you can see we gather up quite a bit of different points in the story.