For many critics, Nelly's sickness is a contrived plot point that is entirely too convenient to be believable; however, most critics concede that sickness was possible if not plausible. Therefore, it does not detract greatly from the narrative and does aid the advancement of Heathcliff's plan for revenge.
Instrumental in Heathcliff's plan is for Cathy to marry Linton, and in order for that to happen, he needs her to care for him. When Cathy discusses her attraction to Linton, her words echo her mother: "he'll soon do as I direct him with some slight coaxing." Cathy, like her mother, enjoys the notion of having control over a man.
