Once upon a time, there was a boy named Joseph. He was the apple of his dad's eye and, as such, became prideful, boasting to his brothers. As a result, his brothers hated him. They sold him into slavery and told his dad he'd died. Joseph grew in wisdom, learning much, and became the head of his master's household. But soon a false accusation landed him in prison.
You know the end of the story. He earned the respect of the guards and was eventually lifted to a position second only to Pharaoh. Joseph went from a prideful boy to a servant leader, but it wasn't without a touch of misery, a nugget of betrayal, and a pound of unmerited discipline.
What does Joseph have to do with writing? Just as God used terrible circumstances to grow Joseph (and us!), so must we use trials to grow our protagonist. This can be hard at first. Hey, we're nice people! We don't like wreaking havoc on others. But havoc must happen for our protagonists to grow, and grow they must.
But don't make too many terrible things happen to your protagonist--this can feel contrived. Sometimes, one solid trial is enough to carry a story.
In "You've Got Mail", Kathleen Kelly falls in love with the man who destroyed the precious bookshop her late mother had opened.
In "The Lakehouse", Kate falls in love with a man who's living in a different time.
In "While You were Sleeping", Lucy finally falls in love, but his family thinks she's engaged to his brother.
In "A Cowboy's Touch", a journalist finds the love of a cowboy and the story of her life only to realize she has to choose between the two.
So bring on bring on the impossible complications, bring on the unfortunate losses, the undeserved consequences. Just remember, it's for their own good--and for the good of your readers.