When it comes to universal themes, these are ideas that humans have been wrestling with for sometime - love, emotions, death, loss, and so forth. In fact, it is this reason that literature falls into the category of the humanities at universities. This is the study of what it is to be a human.
It is not so much that the stories are the same plots, but the stories are addressing many of the same universal themes of mankind. Your job is to create a particular spin on that same idea. If you think of it this way, the universal theme of your story is the thesis. It is the goal of the story and the ultimate message you want to have your reader leave with and understand.
The other way many authors get this confused is the approach that writers take when telling this story. Authors will often weave in common tropes and character types to insure the message gets told. The rake in Regency novels. The hidden information such as a relationship or a baby. All of these are common tropes. Even Shakespeare used these ideas. If you think of Romeo and Juliet, they had character types such as:
- Mercutio - the wild friend
- The Nurse - the clown-like figure
- The hidden information - their secret wedding (which led to all of the problems in Act III, i.
Understanding these elements is even more important when someone wants to write for a series line. The authors don't want writers to copy plot lines, but to utilize common universal themes and tropes that work best for that particular line. This is part of the reason why authors at Harlequin American, for example, use a lot of small town settings and cowboys. These tropes work best for telling the story of the American Dream. But guess what? Thornton Wilder used the same thing when he wrote Our Town. We even see elements show up in Death of a Salesman.
The key here is to understand what message you want to tell and what approach you will take to get the readers to that point.