The Happy Ending
You have to have a happy ending, which in these films means not only the fruition and continuation of the protagonist's love, but usually the saving of the human race as well. These are big, epic stories, after all. Hooking a hot girl up with a hot boy is only 99% of what these films are narratively interested in. They're also about us as a people and whether a girl hooking up with a hot boy can save us from ourselves somehow.