What if Jason found out that he was their enemy, or something? He might come out of his amnesia and decide he hated Piper. He might have a girlfriend wherever he came from.
It doesn't matter, she decided, which kind of surprised her.
Jason always looked so anguished when he tried to remember things. Piper hated seeing him that way. She wanted to help him because she cared about him, even if that meant losing him. (27.28-29)
Just like the choice Piper makes to erase her dad's memory (though she knows he won't be as reliant on her or as close to her afterward), Piper shows her self-sacrificing instinct here. In both cases, Piper is willing to risk her own happiness for those she loves. It's interesting, though, to think about how much it pains Piper to see the people she loves suffer. Does this diminish the degree of sacrifice she makes for them since it also alleviates pain for her?