Themes
Sooner or later we all discover that the important moments in life are not the advertised ones, not the birthdays, the graduations, the weddings, not the great goals achieved. The real milestones are less prepossessing. They come to the door of memory.
— Susan B. Anthony

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Sooner or later, the speaker says, we learn that life's important moments aren't the advertised ones like birthdays, graduations or weddings. The real milestones are quieter and less impressive. They arrive at the door of memory, almost unannounced.
Context reviewed