Many people think the way to write a story is to keep throwing multiple villains and obstacles at the hero. That’s true, but each villain and obstacle must threaten the hero the same way by threatening the hero’s weakness so he or she has no choice but to change.
In “Titanic,” Rose is the hero and her problem is that she feels trapped in life. So everything in the story conspires to make her feel even more trapped in life. First, she’s trapped on a doomed ocean liner so she has no way to escape. Second, she’s being forced into a marriage with a man she doesn’t love, who’s the villain and treats her poorly. Third, her mother has no money and Rose’s upcoming marriage to the villain appears the only way Rose can save her mother and herself from a life of poverty. Notice how so much in “Titanic” threatens the hero in the same way (making her feel trapped)?
Now Rose meets her mentor, Jack, who teaches her to be free by making love to her and painting her in the nude while also showing her that it’s possible to meet a man you can love and who will love you back. Jack represents freedom. Rose also meets other women who represent freedom as well such as Molly Brown, another rich woman who befriends her and also represents another person who has embraced life instead of the limitations of society.
The whole question of “Titanic” is whether Rose will embrace freedom or accept limitations so every part of the story either pulls Rose towards freedom or pushes her towards a loveless life of conformity. That’s what every great story must do.
Think of a single idea and that’s single idea is what all obstacles and villains must threaten. This unified attack on the hero creates a unified story. If you simply have villains attack the hero for no reason, then you wind up with lots of empty action completely devoid of emotion or interest (think “Solo”).
The big question in “Legally Blonde” is whether the hero will learn that she’s strong enough without a man. So all the villains are men who threaten to keep her from learning this starting with her original boyfriend who dumps her (and makes her think she can’t live without him) to her law professor who just wants to treat her like a sex object.
Think of any great movie and you’ll see that the villain and obstacles threatening the hero always attack the same issue in the hero’s life. That’s what makes a great movie, not more special effects or meaningless action (think “Terminator 3”).