In the simplest sense:
There's a guy and a girl. There's tension between them. Readers expect them to realize they love each other. Girl finally brings up the courage and tells the guy she loves him, guy says he loves someone else who lives far away. Girl is sad. Girl runs to other guy. More drama ensues.
I would laugh at this description but alas, has happened to me before.
:C
And why is everyone so depressed?
Put a smile on your face!
:D