Symbolism, Imagery, Allegory
Throughout this book, Lawrence talks about Paul's relationships with Clara and Miriam in romantic terms, and there's nothing weird about that. What is weird, though, is that Lawrence uses many of these same romantic terms to describe Paul's relationship with his mother. Take this example from Chapter 5:
The mother and son walked down Station Street, feeling the excitement of lovers having an adventure together. (5.92)
Now, if Lawrence only used this kind of language to describe Mrs. Morel and Paul's connection once, you could write it off as writerly clumsiness. But as the book unfolds, you realize that Lawrence is really going out of his way to suggest that there's some sort of unconscious sexual deal going on between Paul and his mum.
Again in Chapter 5, Lawrence writes that Mrs. Morel "was gay, like a sweetheart. She stood in front of the ticket-office at Bestwood […] his heart contracted with pain of love of her" (5.86). Later in Chapter 8, we read that Paul loves sitting between his mother and his beautiful "friend" Miriam at church, "uniting his two loves under the spell of the place of worship" (8.233).
The direct comparison between Miriam (Paul's lover) and his mother shows us just how much Lawrence must have been influenced by Sigmund Freud's theory of the Oedipus Complex. This theory basically says that all little boys have an unconscious attraction to their mothers, but they can't acknowledge that attraction, because society frowns on the whole incest thing.
That's why Lawrence takes it upon himself to say the things that his characters can't admit to themselves. He was way ahead of his times in that respect; he wanted to thrust issues of psychology, sexuality, and family into the spotlight in early-20th-century Britain. And yes, people weren't cool with Lawrence talking about this stuff way back in 1913.