Since a memoir is about one's own stories, the theme is based on what part you wish to tell about. It can have different emotions attached to it like something scary, thrilling, serious, happy, or anything which has ever happened that is worth sharing. Most importantly the worth of sharing any incident depends upon how you share it, the author should focus on making sure that the readers are engaged in reading the story just like he/she is engaged in writing and sharing it. The excitement shouldn't be killed in between.