"But we had to celebrate and be glad, because this brother of yours was dead and is alive again

Some things gladden the heart of God, and here he is in festival mode. His vagabond son has come home after squandering his youth afar. A less gracious father would have reproached the son and set about 'making him pay.' Not this father. Nor is he content to celebrate in isolation. The 'well-behaved' offspring is enthusiastically bidden to join the party.