2007 is the 400th anniversary of Monteverdi's L'Orfeo. Monteverdi didn't invent the idea of opera, but his L'Orfeo is the first work worthy to be called a true opera.
The inventors were trying to recreate Greek drama and wanted a Greek subject. Orfeo seemed the right choice. Orpheus represents the power of music to change the world. Rocks dance when he sings. He wins hearts with his singing, but he loses in the larger goal. All of the composers agree that the challenge to return without looking at Euridice is always lost. He can sway hearts but cannot overcome his own lack of faith.
The early operas cannot bear an unhappy ending. In Monteverdi Orfeo joins the gods and is granted immortality. In Gluck Love comes and touches Euridice back to life. It isn't until Haydn that he commits suicide over the loss of Euridice.