In most novels and movies, there are at least two stories going on – the A story (main storyline) and the B story (a subplot). There is sometimes a C, D and E subplot as well, but lets keep this discussion simple by focusing on just two.
Strangely enough, with most love stories, the actual love plot is normally the B story. For example: one of the great love stories was Casablanca. The main story was what was happening to the letters of transit. They were the only way out of a horrid situation, and people were dying to get them. The love story between Rick and Ilsa was the B story.
In writing a plot, the A and B stories wander along in parallel, like two trains going down different tracks, yet racing in the same direction. But at some point usually near the end, the B story must collide with the A story, and affect it in such a way that neither story will ever be the same.
In the example above, Casablanca, the letters of transit fall into Rick’s hands, yet Ilsa desperately needs them so that her husband, Victor Lazlo, can fly to freedom. Ilsa sacrifices everything, promising to abandon Victor and stay with Rick, if he will only give up the letters. Rick, of course, sacrifices Ilsa and gives her the letters out of love, and loyalty. Ilsa and Victor fly to freedom, Rick joins the freedom fighters.
Another example: To Kill a Mockingbird, the A story leads to Tom Robinson’s trial and the hatred of Atticus Finch by Bob Ewell. The B story is Jem and Scout’s developing relationship with Boo Radley. At the end, the A and B stories collide when Ewell tries to kill the children and Boo stabs Ewell to save the kids.
If you have a B story that doesn’t significantly affect the A story in the end, then rewrite it so that it does, or cut the B story. It’s only function is to boost the A story. If it doesn’t, then it’s dead weight.