The novel uses the motif of pairs (of houses, families, and particular couples) to suggest that love and desire don't stay neatly contained—in fact, love defiantly crosses class boundaries, though not without painful fallout.
This motif is visible right away in the contrast between Wuthering Heights and Thrushcross Grange: the neighboring estates are essentially two different worlds, representing untamed nature and cultivated civilization, respectively. At first, it seems like these two worlds can't mix successfully.
That's especially clear in the first half of the novel, with Catherine and Heathcliff's doomed relationship. This couple is ultimately a mismatch because Catherine is raised to be an educated, proper young woman, while Heathcliff is a poor orphan from nobody knows where, who's mostly been allowed to run wild on the moors. More than that, he's been mistreated by his guardian Hindley—causing him to be degraded in Catherine's eyes and an unsuitable match, despite her love for him. It's much more fitting, then, for her to marry Edgar Linton of the Grange.
Yet it's notable that Catherine herself is somewhat like two different people—she contains both "Heathcliff" (Wuthering Heights) and "Edgar" (Thrushcross Grange) sides. After getting acquainted with the Lintons' more refined ways, she "adopt[s] a double character without exactly intending to deceive anyone"—that is, she avoids the rough behavior that she hears the Lintons criticizing in Heathcliff, but she avoids the higher-class behavior at home that she knows Heathcliff will only mock. This dual self perfectly reflects the romantic conflict Catherine faces, and the mingling of different worlds encountered by other seemingly mismatched couples in the novel, especially Isabella Linton and Heathcliff and Cathy Linton and Hareton Earnshaw (respectively cultivated and wild, in both cases).
Yet, so much intermarriage ultimately occurs between the Lintons and Earnshaws/Heathcliffs over the course of the novel that the families and their respective social worlds don't remain totally distinct, and—seemingly by Brontë's design—it's difficult to keep the family distinctions straight. Even though Catherine and Heathcliff's love is doomed, then, their story suggests that class boundaries are artificial, and they only cause generations of needless suffering if they're allowed to carry too much weight.
The novel uses the motif of the afterlife in different ways to symbolize nearness or distance from one's beloved. Heaven can either be a place where one is a stranger because one's beloved isn't there, or it can even be identified with the beloved herself. Paradoxically, then, heaven can be hellish, while hell (or the loss of heaven) can be a joy.
In Chapter 5, after Mr. Earnshaw dies, Nelly overhears young Heathcliff and Catherine comforting each other: "no parson in the world ever pictured Heaven so beautifully as they did, in their innocent talk." At this point, the idea of heaven is conventional—it's a beautiful afterlife, and this is a consolation to the bereaved. The innocence dissipates, though, as the two grow older and develop romantic feelings for each other.
In Chapter 9, an older Catherine tells Nelly of a dream she had about being in heaven and hating it there, because she was separated from Heathcliff on earth:
"I broke my heart with weeping to come back to earth; and the angels were so angry that they flung me out, into the middle of the heath on the top of Wuthering Heights; where I woke sobbing for joy [...] I've no more business to marry Edgar Linton than I have to be in heaven."
Linton is associated with the goodness of heaven—a goodness Catherine has "no business" claiming and ultimately doesn't want. Most people would be joyful in heaven, but in her dream, Catherine despairs, suggesting that she's wicked and unworthy of staying there (hence the angels' anger). The reason Catherine weeps is that Heathcliff is on earth, and Catherine wants to be earthbound with her beloved, even if it means being "flung out." Here, the motif underscores the strength of romantic passion. It suggests that marrying Edgar Linton would actually be good for Catherine, but she would rather have an illicit passion than a safe, conventional love.
The motif recurs in Chapter 34, when Heathcliff is approaching death. Watching Cathy and Hareton's burgeoning romance, Heathcliff feels tormented by Catherine's spirit, but as he gives up on revenge, the torment becomes a joy to him:
Last night, I was on the threshold of hell. To-day, I am within sight of my heaven—I have my eyes on it—hardly three feet to sever me!
"My heaven" is, of course, Catherine herself—and Heathcliff believes he has to die to be with her again. On the other hand, hell is being stuck in life with nothing but tormenting reminders of Catherine (like Hareton and Cathy). In the context of love and passion, then, the afterlife has more to do with the proximity of one's beloved than with traditional conceptions of good and evil.
The novel uses the motif of the afterlife in different ways to symbolize nearness or distance from one's beloved. Heaven can either be a place where one is a stranger because one's beloved isn't there, or it can even be identified with the beloved herself. Paradoxically, then, heaven can be hellish, while hell (or the loss of heaven) can be a joy.
In Chapter 5, after Mr. Earnshaw dies, Nelly overhears young Heathcliff and Catherine comforting each other: "no parson in the world ever pictured Heaven so beautifully as they did, in their innocent talk." At this point, the idea of heaven is conventional—it's a beautiful afterlife, and this is a consolation to the bereaved. The innocence dissipates, though, as the two grow older and develop romantic feelings for each other.
In Chapter 9, an older Catherine tells Nelly of a dream she had about being in heaven and hating it there, because she was separated from Heathcliff on earth:
"I broke my heart with weeping to come back to earth; and the angels were so angry that they flung me out, into the middle of the heath on the top of Wuthering Heights; where I woke sobbing for joy [...] I've no more business to marry Edgar Linton than I have to be in heaven."
Linton is associated with the goodness of heaven—a goodness Catherine has "no business" claiming and ultimately doesn't want. Most people would be joyful in heaven, but in her dream, Catherine despairs, suggesting that she's wicked and unworthy of staying there (hence the angels' anger). The reason Catherine weeps is that Heathcliff is on earth, and Catherine wants to be earthbound with her beloved, even if it means being "flung out." Here, the motif underscores the strength of romantic passion. It suggests that marrying Edgar Linton would actually be good for Catherine, but she would rather have an illicit passion than a safe, conventional love.
The motif recurs in Chapter 34, when Heathcliff is approaching death. Watching Cathy and Hareton's burgeoning romance, Heathcliff feels tormented by Catherine's spirit, but as he gives up on revenge, the torment becomes a joy to him:
Last night, I was on the threshold of hell. To-day, I am within sight of my heaven—I have my eyes on it—hardly three feet to sever me!
"My heaven" is, of course, Catherine herself—and Heathcliff believes he has to die to be with her again. On the other hand, hell is being stuck in life with nothing but tormenting reminders of Catherine (like Hareton and Cathy). In the context of love and passion, then, the afterlife has more to do with the proximity of one's beloved than with traditional conceptions of good and evil.