whose fault?

a story about middle school friendship of a boy named Arthur, he has a friendship whom he consideres like family. especially that one friend, no, bestfriend, his bestfriend just likes his own big brother, their bond seems unbreakable but something happened between them and it makes a conflict between them Arthur thought it was his bestfriend's fault, so does his bestfriend. Let's see how was their friendship created and how is it going untill the conflict happens