A story's moral is the lesson it teaches about behavior that is right or prudent. Short works, especially fables and folk tales, are usually said to have morals (lessons taught about good and wise behavior) because their length, structure and form conventions lend themselves to single, well defined lessons. Long works, like novels, plays and movies, are usually discussed in terms of themes and motifs because their length, structure and form conventions are complex and allow for multiple lessons that are stated as a collection of themes, with an overarching theme that is the dominant focus of the novel. Morals are easily stated as adages, like "Value the beauty in what one posses," while themes are more complex and may involve lengthier statements, like "When society errs in the directions of two opposing extremes, humanitarian solutions to social ills, though difficult to find and implement, are still possible." With this in mind, and with the understanding that a long work like David Copperfield, with multiple subplots covering several decades of narrative time, are usually discussed in terms of complex themes, it may be possible to distill the essence of the novel into one, simply expressed moral lesson.
What is the crux of the import of the narrative in David Copperfield? This autobiographical account of David Copperfield (grown out of a failed attempt at an autobiography by Dickens himself) describes Victorian mistreatment of children and women. Both sides of each issue are developed in antithetical characters. Steerforth is dangerously over-indulged rendering him selfish, calloused and amoral, without effective moral compunction. David is over-denied and over-deprived, putting him at the mercy of undesirable people of varying degrees of undesirability. Dora is over-pampered, rendering her useless and silly. Agnes is over-idealized, putting her beyond normal reach and elevating her to responsibilities beyond her years. Dickens ends by suggesting humanitarian remedies to social ills (e.g., emigration from the oppressive society to a liberal, generous one).
From this we might derive a single-focus story moral. The moral of the story might be said to be this: The outcomes of opposites kinds of errors can both find solutions in fresh starts. Another possible single-focus story moral might be: Don't be naive and don't trust falsely based humility, affection, friendship, or love.
'And now tell us,' said I, 'everything relating to your fortunes.'
'Our fortuns, Mas'r Davy,' he rejoined, 'is soon told. We haven't fared nohows, but fared to thrive. We've allus thrived. We've worked as we ought to 't, and maybe we lived a leetle hard at first or so, but we have allus thrived. What with sheep-farming, and what with stock-farming, and what with one thing and what with t'other, we are as well to do, as well could be. Theer's been kiender a blessing fell upon us,' said Mr. Peggotty, reverentially inclining his head, 'and we've done nowt but prosper. That is, in the long run. If not yesterday, why then today. If not today, why then tomorrow.'
'And Emily?' said Agnes and I, both together.
'Em'ly,' said he, 'arter you left her, ma'am—and I never heerd her saying of her prayers at night, t'other side the canvas screen, when we was settled in the Bush, but what I heerd your name....