Answer (don't copy and paste, use your own words):
The memories of the past affect Jonas's relationships with his family and friends because he now feels real emotions. When Asher, Fiona, and other kids were playing a game of good and bad guys fighting each other, it related to Jonas's experience of warfare, and he remembered the pain he and everybody else went through in war. When Lily was angry, she wasn't really angry. Jonas knew what real anger was.