I'd like to highlight some classic storylines in literature so that you can see how to incorporate them into your own fiction. So let's begin with one of the oldest classic storylines, the star-crossed lovers!
First, let's start with a misconception. Many people think this term, appearing most famously, of course, in Shakespeare's Romeo and Juliet, refers to lovers that are destined to be together. But actually, it's just the opposite. Let's go to the famous line:
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes
A pair of star-crossed lovers take their life,
Whose misadventured piteous overthrows
Doth with their death bury their parents' strife.
In this opening passage from the play, Shakespeare lays out the entire plot, as was traditional at the beginning of plays in his time. Here, the star-crossed lovers take their lives; and star-crossed refers to lovers who are doomed, tragically, to stay apart. Romeo and Juliet are the most famous examples, but they themselves were a rip-off of Pyramus and Thisbe. And today, many of the novels we love best have lovers that seemed not destined to be together, but doomed to be parted. We can see this in the fiercely loved novel Gone with the Wind, among many, many others.
Using star-crossed lovers.
So why do we find this literary trope so powerful? The fact that the lovers' story ends in tragedy seems both more real and more poignant to us; we tend to be moved more by a story of thwarted love. A lesser writer might be tempted to satisfy his or her own need for wish-fulfillment and put the lovers together, but a disciplined writer knows that they must be separated for us to keenly feel their emotions. The lesson we learn when we see star-crossed lovers is about love, and life. We learn that love is often a state of loneliness, and we learn that life is not always fair, and sometimes innocent people die for no reason.
In your story, remember that you are trying to capture some aspect of reality, not wish-fulfillment or fantasy. That's a crucial difference, I believe, between high and low-quality fiction. Remember how you felt when you were emotionally wrenched by tragic-love story, and try the hard choice of keeping your characters apart. It's a quick but powerful way to make your readers pay attention and feel more deeply.