Stargirl changed for Leo. She really liked him and she wanted him to be happy. She began using Susan, dressing like everyone, talking like everyone, she tried to change who and what she was for Leo. She knew that being accepted by "them" was important to him, so she tried. However, as always happens when we commit drastic changes for anyone other than ourselves, something happens to bring us back to who we really are. When Susan arrived home from the speech competition expecting to be welcomed with open arms, and no one was there but Dori, Susan realized she could never be what "they" wanted her to be so she returned to Stargirl.
As Archie tells Leo on page 178,
"She did it for you, you know."
"Gave up her self, for a while there. She loved you that much. What an incredibly lucky kid you were."