David falls in love with his boss's daughter, Dora Spenlow, at first sight. And, quite frankly, we're not quite sure why.
...
Dora's father, Mr. Spenlow, is unaware of their courtship...and demands that David forgets the whole thing. But then, Mr. Spenlow suddenly dies...So, while Mr. Spenlow is alive, David can't marry Dora. And once Mr. Spenlow dies, David immediately (and symbolically) takes Mr. Spenlow's place as Dora's caretaker. He spoils her and fusses over her as though her were her father as well her fiancé. And it's, well, a little weird. 

...

[David is] such a young man himself that he's not ready to be married to such a dependent woman, in need of being spoiled and cared for all the time the way Dora does. Still, despite these concerns, Dora and David get married. 

...

Dora miscarries and slides into the long illness that will eventually kill her...the whole Dora episode leaves a bit of a bad taste in our mouths...

https://www.shmoop.com/david-copperfield/dora-spenlow.html

And that's the other thing: Dickens does not even kill Dora good. The miscarriage did not kill her, a "long illness" killed her. What is this "long illness"? She's a teenager! Healthy as a horse! Brains to match. 

I agree with everything "Shmoop" wrote there. Dora is a ridiculous, superfluous character who Dickens should have aborted rather than bring to life and then unconvincingly snuff out.

I have had this thought before with Bleak House, even researched it a bit one time. These long novels get away from Dickens. There are sooo many characters, so many barely overlapping sub-plots I wondered if he wasn't on some stimulant that over-stimulated him.

Another example is Mr. Dick, next.