When we think of a story being a mystery the tendency is to think of the mystery genre. An investigator (usually a detective), a puzzle to be solved (usually a crime), a person to be caught (usually a criminal).
But to all intents and purposes every story is basically a mystery. There is always a burning question that needs answering and someone who is tasked with finding that answer. It’s just that it might not be as obvious what the question is in Looking for Love as it is in Who Killed Johnny?
And if it’s well written the reader’s desire to also discover the answer should be just as strong in both stories. Which is why when that desire isn’t so strong we can use the mechanics of the mystery genre to help work out what’s gone awry in other types of stories.