Why does Rick state more than once that this is not an important book?

Rick claims that the book is not important because it is just about the lives of one American family. Not a good life, a hard life, and so it doesn't stand for much. However, the book is important for his mother to understand that she was not a failure, and so in writing such a book it becomes exceedingly important for all mothers who feel this way and for the rest of readers to understand what others have to go through sometimes.