Schala (Magus Refusal)
Schala coldly rebukes Magus at the end of Chrono Trigger DS, telling him that he can't save her from the Dream Devourer, and that he should dwell on her predicament no longer and return to his own time. This was a tragic moment, as Magus had heretofore devoted his life to saving his sister. Why did Schala say these things?
Schala is likely aware that she can't be freed from the Dream Devourer without a power beyond anything that had yet existed in the timeline (the "Chrono Cross", which at that stage was still merely a dream on Belthasar's drawing board). Physical strength and magic alone could not defeat the Dream Devourer (or as demonstrated in the bad ending of Chrono Cross, not defeat it without also destroying Schala). She knows at the ending of CT DS that her brother cannot save her, and that his determination is so great, he will never stop trying.
So she does the one thing she can do to save him: break his spirit, tell him his quest is hopeless, that he cannot ever save her. Schala is giving him a chance at his own life that isn't tethered to her or Lavos, something he never had. And Magus actually listens. Whether she ultimately foresaw this or not, Magus willingly gives up his memories. His determination would destroy him otherwise, and his pride is probably still too great to ever allow that to happen. Assuming he becomes Guile, he does live his own life. He becomes his own person, not tied to Schala or Lavos or Zeal. He becomes accomplished, rising to the top of a magical guild. He makes friends. As Guile, Magus is happy. And I don't think Schala, at the end of Cross, wants to ruin that life she gifted him by restoring his memories, when she, again, wants to live her own life through time.
Thanks to Kodokami