The best (non-celebrity) memoirs I've read follow a theme, as MaeZe says, and follow that theme along a story arch. Memoirs written by former local athletes coming to terms with plain-Janeness after leaving high school, people becoming adults after a sheltered or dangerous childhood, men rebuilding after divorce, and addiction recovery memoirs which is almost a genre unto itself.

In each case, the point is not an all-encompassing synopsis of the author's entire life, but their progression through (i.e., their story arch) a particular aspect of it from where they start to where they end. Keeping in mind that "all symphonies remain unfinished," and both the author and reader recognizing the author will continue to grow. At the end of Tara Westover's wonderful memoir Educated, I really hope she writes a followup chapter or two in twenty or thirty years time. She made me care not only about what she experienced, but her internal growth not only up to The End, but beyond.