Cathy says the only reason she even considered marrying Linton was to help Heathcliff. She explains to Nelly Dean that she can't marry Heathcliff now, despite her deep love for him, because Hindley has degraded him too far by turning him into a farmhand. If they were to marry, they would have no money. However, if she marries Edgar, she can keep Heathcliff close to her, but, more importantly, Edgar can use his connections to help Heathcliff get ahead in the world. As Cathy puts it:
"Nelly, I see now you think me a selfish wretch; but did it never strike you that if Heathcliff and I married, we should be beggars? whereas, if I marry Linton I can aid Heathcliff to rise, and place him out of my brother’s power."
When Nelly says that Edgar may not be interested in helping Heathcliff or having him around, Cathy passionately explains that he must learn to tolerate Heathcliff, for her life is worthless without him:
"Every Linton on the face of the earth might melt into nothing before I could consent to forsake Heathcliff. Oh, that’s not what I intend that’s not what I mean! I shouldn’t be Mrs. Linton were such a price demanded!"
Nelly scolds Cathy for marrying a man she doesn't love for the sake of the one she does and warns her that the future may not go as smoothly as she thinks it will. Cathy, however, is undeterred.