The narrator of Jude the Obscure sees a lot but doesn't seem to feel much. That is, the narrator knows everything about everybody: major characters, minor characters, you name it. But the narrator also has a sense of humor, keeping up a steady series of snarky comments on the events of the novel. And even though the events of the novel are pretty tragic, the tone of the narrator's light-hearted jokes never really changes.
We find that the narrator's distance from the characters—the narrator's ability to make objective, even funny comments about them while they are suffering—only increases our sympathy with the flaws and weaknesses of Jude, Sue, and all the rest. Take, for example, this passage when Sue has just realized that people are spreading reports that she and Jude are sleeping together and that they should get married to save their reputations:
By every law of nature and sex a kiss was the only rejoinder that fitted the mood and the moment […] Some men would have cast scruples to the winds, and ventured it, oblivious both of Sue's declaration of her neutral feelings, and of the pair of autographs in the vestry chest of Arabella's parish church. Jude did not. (3.5.47)
First off, the narrator gives us a giant overview of "every law of nature and sex": talk about all-knowing! This kind of big talk proves the narrator's sweeping understanding of the world of the novel. We're not supposed to doubt that what the narrator is saying is true.
Second of all, the narrator is using Jude's hesitation to kiss Sue, even though "some men would have cast scruples to the winds, and ventured it," to tell us something about Jude's basic character. Jude is a deeply principled guy. Sure, maybe Jude hasn't told Sue about his marriage to Arabella yet, but his respect for Sue's wishes when it comes to kissing and sex and all the rest of it demonstrates that he is a decent, stand-up person whose "scruples" (which are doubts about the morality of something) stop him from pushing her.
Lastly, though, we are struck by the "Jude did not" aside. The narrator sets up this huge expectation for what Jude might do—and then the narrator suddenly lets us down with three words: Jude did not. The narrator's phrasing here makes us laugh a bit, in spite of the seriousness of the situation, because it's such a simple, brief response to a paragraph that begins so big.
The narrator's statement here really emphasizes how unusual Jude is: all of the world would kiss Sue, but not Jude—no siree. Because (and here's the joke) Jude is a unique weirdo.
The narrator's emotional distance from Jude and the other characters gives us some much-needed space to absorb the horrible things that happen to these people in the plot. But it also makes us feel that much more pity for Jude. Even the narrator, in all his objectivity, notices that Jude is always going against the tide of the rest of the world, that he never makes the obvious or the easy choice—which is why Jude just keeps suffering so much throughout his life.