In my opinion she's never really felt what it's like to say "I love you" or what it means to "love". Maybe she's never felt confident to love someone. She's scared that he might take a second thought and leave her when all she's done is fallen in love with him. Overall, she's being careful and watching out for herself. "I love you" is something or some words you just put out there, those words can be the most beautiful thing to say to someone