Warning: contains spoilers for the following films***
Avengers: Infinity War
Into the Woods
The Little Mermaid (both original tale and Disney movie)
Tarzan (both original tale and 1999 Disney movie)
When people watch a movie, they not only expect a good story, but also a lot of conflict. Because every fictional work, visual or written, needs problems and obstacles, many audiences expect satisfying endings, where the protagonists achieve their goals. If they don’t, then it’s either because they realized those goals weren’t what they wanted all along or they weren’t the right kinds.
Rarely these days are film endings unsatisfying. One example includes “Avengers: Infinity War”, where the characters dissolve into dust.
The other example of a movie with an unsatisfactory ending is “Into the Woods”. The baker’s wife dies and so does the witch after her curse was undone. I understand that the point is that there is no such thing as “happily ever after”. But it was still unsatisfying, especially coming from Disney. Disney is known to sugarcoat their film endings.
Like in “The Little Mermaid”, where Ariel marries Prince Eric as a human. In the original tale by Hans Christian Andersen, she dies at the end and the prince gets another bride. I read that the creators of the Disney version found that sad. They felt that in order to make their adaptation more kid/family-friendly, the ending needed to be happier.
Another example, although probably not as sad (I could be wrong, though), is Disney’s “Tarzan”. While Jane and Professor Porter join Tarzan in the jungle, in the original novel, Tarzan is brought back to civilization.
Audience and genres also contribute to how the films should end. Horror films often end a certain way. The ones I saw didn’t have very satisfying endings. But there likely is a purpose for that. Horror movies are not intended for families or children. Kid-friendly films have different standards from mature movies, besides cleanliness. Also, many young people yearn for happy endings. So do older crowds.