>>279720540
>>254506780 eloquently describes what I wanted to say. My strongest belief about the show is that the Haibane are humans that died before they were meant to. Either due to accidents, illness or voluntarily giving up on their life. They end up in Glie as a sort of Purgatory where they can live the life which they missed out on. Once they are satisfied with their life, either by fulfilling their wishes or by severing whatever attachments they had with the human world. they have their day of flight.