From Vladek's perspective as a survivor of the Holocaust, the word "friends" doesn't mean much. Vladek saw friends and family betray each other in order to save their own lives. When it really comes down to human nature, it's a dog-eat-dog world. Vladek always lived his life in the way he would like to be treated, but he also realized that not everyone can be trusted, not everyone is the same, and not everyone can be a true friend. Surely, Vladek thought less of Artie's friends for leaving him just because their friend fell down than those who left someone while running for their lives, but Vladek was teaching his son a principle. A person can't awlays stop life to feel sorry for himself just because of the way his so-called friends treated him. If a person wants to live, s/he needs to figure out a way to keep going despite what other people do. Had it been Vladek who fell down while rollerskating, he would have gotten back up on those skates and out-raced the friends; or, he would have gotten back up and skated his own way without worrying about his "friends." Vladek is a survivor because he didn't worry about what other people did or said; he simply found a way to keep going.