Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third-Person Omniscient
It's always nice when a writer establishes his or her narrative perspective in the very first line of a story. It's like, so considerate to us readers, you know? In Sons and Lovers, Lawrence does this exact thing, opening with an overview of the Morels' neighborhood and its recent history:
Then, some sixty years ago, a sudden change took place. The gin-pits were elbowed aside by the large mines of the financiers. (1.2)
This eagle-eye view of both local geography and history tells us right away that we're dealing with a godlike, omniscient narrator.
We also find out that this narrator not only knows everything about the outer world of the book, but also the characters' inner worlds. A good example of the narrator's all-seeing powers can be found in Chapter 6, when the narrator says:
[Paul's] heart was full of happiness till it hurt. His mother had to chatter, because she, too, wanted to cry with happiness. (6.264)
In this brief line, the narrator displays knowledge of what's inside Paul's and his mother's heads, though neither character says anything.
Since Sons and Lovers is mostly about one character, Paul Morel, it would have made sense for Lawrence to write the thing from a first-person perspective. But by choosing an omniscient third person narrator, Lawrence positions the Morels' problems inside the larger historical conflicts of modern industry—e.g., the English mining industry that graces the book's opening pages.
Also, it is important to remember that one of the main themes of Sons and Lovers is the fact that people never just come out and say everything they're trying to communicate to another person. So Lawrence's internal insight into the book's characters allows us to witness the similarities and differences between what is said and what remains unsaid.
Plus, the third-person omniscient narrator allows Lawrence to make us a little more sympathetic toward evil or pathetic characters like Walter Morel, whom every other character seems to hate. And for Lawrence, having a little compassion for any person is a good thing—especially in a modern industrial world that does its best to make us hard and cruel.