Jesus justifies having a relationship with sinners because of the joy! To retrieve what was lost is joyful. The celebrations are significant enough they can't happen in isolation; they are community, heavenly celebrations. The searching is an individual responsibility, the celebration is a community activity.
The first two stories focus exclusively on the joy, as does the first half of the third. All the joy sets us up for the anger of the older brother. It is the opposite of a happy response. Oh the pain of trying to earn God's favor. The older brother neither loved the father nor did he enjoy him. All this because the Pharisees were grousing about him loving sinners.