A good short story, personal essay, poem, or memoir captures the texture of life. The most celebrated memoirs are those which tell more than one story. For example, in Tobias Wolff's This Boy's Life we read about the boy's education and conscience, but also about the bad second marriage his mother is trapped in, and how she is changing, doing things formerly out of character, like campaigning for JFK. These experiences happened during the same period. They were parallel.
Wolff could easily have written the entire memoir about his youthful self and how he learned to lie and fight. He could have written a fine memoir just about his mother's life. Either one would probably have been swell. But these in reality were intertwined, and Wolff wrote them that way.
What this appears to accomplish:
1) More accurately depicts the boy's exterior reality (events, conversations, his stepfather's behavior, friends and schooling and lessons learned)
2) More accurately depicts his interior reality. We all live more than one life at a time. In fact, at least two: the life that people can see and the one they don't. I've read (dull) stories and essays delving deep into an individual's emotional life that never indicate that this character or person has a job, or siblings, or a loan to pay off, or a best friend who isn't a dog, or a political opinion, or a goal.
Fiction and nonfiction have this in common: To capture the texture of real life, the work needs a subplot or more than one narrative thread.
You can see this on television, say, on The Simpsons, when the main story is about, for example, Homer, but a secondary story is woven in about two other characters. If you look for this, it is absolutely everywhere. That's because having two or more threads captures the texture of life.
When your creative prose seems dull or flat or thin or like "weak tea," it's usually because it has only one facet or thread. A secondary or parallel story, or "subplot," is a lot of work for the writer and requires skill. It is a large part of what makes superior fiction and creative nonfiction. You can spend years in creative-writing courses and never once hear about subplotting, or why subplotting is as basic as the "main story." I have, however, heard a poet say, "A poem should always be about two things." Poets get it.
Prose writing is a little different. After you have learned how to develop and play on one thread, attempt to add another to the piece you are working on. Don't worry about how well or poorly you do it at first. I said it's a skill and that it's not easy.