A shepherd boy was bored of just watching sheep, so he cried out that
there was a wolf, and the people from the nearby town came running to
save him, but were angry when he laughed at them and called them all
fools. The next day a wolf really came, but nobody listened when he
cried out, thinking it was another joke at their expense, so he was
The reason people often react poorly to the question of how to make a story longer is that we all have bad experiences with "filler" chapters and episodes and scenes, which really feel like their only purpose is to get to a required length-quota. But this seems like a sincere question. Let's build from the story above.
The fundamental theme of the boy who cried wolf could span a brief paragraph, or it could be much longer. How much longer? I've seen it rendered as a couple of pages. If I kept only the central idea, I could likely stretch it out to fill a novel. At that point I probably wouldn't be writing about a literal wolf or a literal shepherd, but perhaps about a young accountant who rises to importance by (incorrectly) alleging corruption and destroying someone else's career. Later he discovers real corruption, but it the midst of the new investigation, the skeleton of him being a liar (or maybe only careless?) in the previous investigation destroys his reputation, which actually saves the villain he was pursuing.
Throughout this framework could be woven the courtship, marriage, and budding family life of this accountant and his high-school sweetheart, who he'd met again unexpectedly - and how the woman he cares for is tragically dragged down with him. There would be relationships with coworkers, too - perhaps tainted from the beginning by the main character being constantly nervous that others might realize that maybe the incident that raised him to importance in the first place could someday turn and destroy him, as it eventually does.
If I took 300 pages to describe a bored shepherd-boy's afternoon, and a crowd of people running to his rescue - and then not running to his rescue - it's unlikely that I could put in enough details to keep even the most charitable reader's interest. You could stretch the account's story that far.
In The Neverending Story, by Michael Ende, the Grograman talks about having just come into existence when Bastion wished for a companion, but also having existed for thousands of years. While causality in-story should make sense and be linear, the cause of a character's existence, or a problem, from your point of view as the author, is to make interesting tension, and to serve to build towards whatever it is your story is about. Events in a story must serve some general idea, though, or they're off-topic and don't belong. Nobody cares if the shepherd boy was even wearing a watch, though what kind of watch the doomed-for-the-fall accounted wore might say something about his character, and then it would be fair game).
If your stories get to the point too quickly, you do need to weave in something else. But the details must serve the story you're telling. And not every telling of every story will bear as much digression or detail.