Peter was quiet for a moment, letting her words sink in. The magic had chosen her. It made sense to him. Sarah was human, she had been subjected to the Labyrinth - twice - and both times her will had been stronger. Watching her in that place, Peter has seen the kind of connection she had to it, the simple acceptance that had started the avalanche of his own love her conveyed into everything and anything that place had been able to throw at them.
It had chosen her. To take Jareth's place. That was the key. It had chosen her. Not him.
Someone had to have it. Peter could all too well imagine what would happen if 'things fell apart'. Magic needed a source. A kingdom needed a ruler, just as a city needed a champion.
Peter put his hands on her shoulders, squeezing gently to help ground her. "Sarah," he said softly. "I have to ask something. And I just need a straight, simple answer. Okay?"