Couples as with singles are more interesting in fiction with adversity and problems to work through
adversity, yes. Friction...
Ever see the movie, "Undercover Blues"? Kathleen Turner and Dennis Quaid play a pair of spies trying to retire, because they have a new baby. Of course, there has to be one last case...
Their relationship makes the movie. But at no time are they in friction. I suppose for a series it would be harder to maintain interest...