If a story keeps introducing new elements, then you risk overwhelming the audience with so much information that doesn’t have any connection or relation earlier information. That’s why it’s far better to parallel story structures. That means you repeat the same story structure in multiple ways.
In Disney’s animated “Tarzan,” the gorilla mother loses her baby when a leopard kills it. Then this same leopard kills Tarzan’s parents. Later, Tarzan kills this leopard and offers the dead body to the gorilla leader as a way to make up for the gorilla leader losing his own son to the leopard. When stories link and parallel each other, it reinforces the story and helps us remember the significance of earlier information. If every part of the story were completely different, then the story would feel more like just a collection of random scenes.
Music is very structured and repeats itself. Any song that doesn’t repeat itself risks degenerating into random noise. Movies and stories are no different.
In “Legally Blonde,” the hero wants to find love. When she meets a hairdresser, that hairdresser also wants to find love in the cute UPS deliveryman who drops by periodically. The hero also runs into a law professor who fails in love with the hero. This also works in a negative way too when another law professor just sees the hero as a sex object. With so many characters searching for love, the story feels more coherent and unified.
In “Wonder Woman,” Wonder Woman wants to do what’s right, which means risking her life to protect others. In a similar manner, her boyfriend, Steve Trevor, also risks his life doing what’s right to protect others, so they both have similar goals.
In “La La Land,” the hero is an aspiring actress who wants to hit it big in Hollywood. Her mentor is an aspiring musician who wants to own his own nightclub one day. They both have similar dreams so their stories feel consistent. Imagine if the hero were an aspiring actress but her mentor wanted revenge against a man who killed his father. Suddenly their goals would be wildly different so their stories wouldn’t work together and would instead interfere and distract from each other.
Sow hen writing your own story, make everything parallel. Stories should be more alike than different between all your characters. When in doubt, make every character pursue similar goals. You’ll find it helps create a stronger, more unified story in the end.