Rikku used to think of life in simple terms. You were either alive or you weren't.
Then she met Auron and realised that life was more complicated than that. Auron was dead and had been for 10 years but he was alive. He was harsh, refusing to coddle her in the Thunder Plains. He was kind, saving her from Wakka's wrath on the frozen lake. He was comforting, letting her cry out her grief on his shoulder as the airship left the burning remains of Home behind. He was everything to her.
When Auron died, really died, Rikku realised that death was more complicated than that. She was alive and, fayth-willing, would be so for many years to come but she was dead inside. She was numb, trying too hard to be the merriest, to be Rikku. She was lonely, using his swords just to keep a small part of him by her side. She was lost, searching Spira for an answer, a way to bring him back. She was nothing without him.
Rikku thought of life in simple terms. To live meant to be without him, to die meant to be with him.
Out on the airship's deck, the wind blowing through her hair, the choice was easy to make. She closed her eyes and let herself fall.