Protip: write a shorter story.
When you don't have much to tell, don't try to make a story longer than it is. Some stories naturally demand more content, while others... not so much. What adds depth to a story is its themes, and the exploration of those themes by putting your characters in different situations and seeing how things play out. If you don't have much to tell, it might be because you're not writing based on a theme, but rather based on events that play out in your head(in which case, your story will be a straightforward narrative 90% of the time).
One thing I'd recommend is reading and researching widely in the genre. I also recommend studying the scientific mechanics of falling in and out of love, the psychology of all emotion and its ebb and flow, and how people actually work. A lot of times as writers, we can only write about what we know - so if you don't have much life experience, it might be because you're not going out and getting that life experience... or, conversely, living out someone else's life experiences through a book. This can be done easily by reading biographies, as that genre (in particular) can help you understand different kinds of humans and their inner worlds. The genre allows for - dare I say - a deep form of voyerism that peers directly into someone's soul.
Finally, simply write. Write short stories, write long stories. As you write, you'll understand how stories are structured and how to add more content(if necessary, that is - padding out is NEVER advised).
Here's an exercise: take a romantic concept(first date, unrequited love, celebrity crush... whatever you like to write about) and try writing it out in a single scene. Take only the barest essence of it, and describe it to your fullest ability. As you keep doing this, you'll understand how depth is not about padding out your stories, but rather by adding a lot of individual parts that come together to form a greater whole, a greater narrative. You need a grand theme(for example, "love" instead of just "celebrity love", or "loss" instead of "loss of a family member") to add more depth, and you can do that by mastering the exercise of writing about a single topic in the least amount of space required.
Hope this helps you. Peace out.
