Why is Miranda worried about Megan, and how do Megan's actions make Miranda feel guilty?

When Megan begins giving away a portion of her food to another student each day, Miranda confronts her. She doesn't think it is sensible for Megan to do this, since she needs the food herself. Megan claims that she only needs God's love to sustain her, and is more concerned about being a good person than about staying alive. Miranda sees this as a threat to her friend, but perhaps also feels very guilty because she is not doing the same. She has more food than the others, but still keeps her own portion for herself. She worries that she would not be able to give up food for her own brother if needed, and seems to feel that she is not as good a person as Megan.