on Writers Helping Writers:
Whether you are writing romance, adventure, fantasy, or mystery, nearly every well-told story has a relationship plotline. Unfortunately, though, entertaining banter or fiery arguments alone aren’t enough to sustain that plotline.
Your characters may be love interests, friends, neighbors, allies, rivals, or even enemies, but regardless, they should work off the same basic storytelling principles.
They should have a relationship arc, and they should have the proper relationship plot elements in play.
Chris The Story Reading Ape's Blog 