Here's a few I've noticed that authors use (no spoilers):
One is human and one is not. That usually creates problems. Vampire and human, angel and human, alien and human. And it even ramps it up even more if they're enemies, like angel hunter and angel or dragon hunter and dragon.
Forbidden love. This can happen because they're different species (like above) or because of some other reason, like student/teacher or trainer/trainee, guard and queen/princess, slave/slave owner, etc.
From different worlds. Not literally, but examples would be a big class difference or a caste system. This happens a lot in dystopian and fantasy. Think Day and June from Legend by Marie Lu or Aria and Perry from Under the Never Sky.
The rules/government. They would be together, but they're not allowed to be. And it may even be that they are on the opposite sides of a war or some sort of struggle.
And then there are more specific ones:
They're attracted to each other but find out they're brother and sister. Gotta love that one.
They would get together but if they do, but he's been cursed so whoever loves him will die.
Can you guess which books these are??
Then, if they do finally overcome whatever it is and it's a series, usually something happens after book one (this is typical book 2) that drives a wedge between them. Some examples are:
Misunderstandings. Their love is new and fragile and yes, they've overcome whatever the main obstacle is but usually they don't know each other very well and the reason that kept them apart rears its ugly head and leads to misunderstandings.
The outside world. Although they've overcome whatever prejudices and problems exist, they are now having to deal with everyone else and the reaction to them being together, and that creates problems.
A job to do which leads to being physically apart. Sometimes the author has the couple have to split up in order to deal with the war or whatever the problem is.
Death (at least that's what he/she thinks).
Kidnapping. Or sucked back to hell. Something like that.
Love interest. The dreaded love triangle. Usually another guy comes into the picture because there is a misunderstanding or the outside world has created problems or they're physically apart or the girl thinks her love is dead.
This is obviously not an exhaustive list, but these are some of the main ways I've noticed authors keeping couples apart. What are your favorites? What other ways have you noticed?
Posted by: Pam