It's rude to judge people who watch anime.
John meets Bill for the firs time and in introducing himself Bill mentions that he enjoys watching Anime.
John wants to respond to Bill's statement.
John tells Bill that he doesn't really enjoy watching Anime himself but that it's nice that Bill has something he loves, which is important in life.
John tells Bill that Anime is basically a cartoon and cartoons are for children and adult idiots.
Bill is happy that John seems like an understanding fellow and is eager to get to know him better.
Bill feels incredibly disrespected and also begins to feel sad and insecure about his hobby.