We've all heard the saying: opposites attract. This is usually talked about in the real world, but it's also the same in the fictional world.
Lately, I've been watching a few different sitcoms on Netflix--all with eight or more seasons--and I've noticed how opposite many of the characters are. For example, on the sitcom Family Ties, the parents we former hippies from the 60's who were about as liberal as they came. Their son Alex P. Keaton, on the other hand, was a lover of Nixon and Reagan and was very right-wing. In Wings, the two brothers who end up running the small airline were opposites: one was methodical and orderly while the other was carefree.
In your writing, look for unique ways in order to make opposites attract. What if a girl born into the upper crust of high society fell in love with a boy who lived in the backwoods? What if a small town cop was working a murder case and had to investigate it beside a high-charging FBI agent?
Examine your characters on a deep level and draw out these opposites. Not only will they make your charactesr more real, your readers will relate to them better.