Cathy and Hareton have fallen in love at the end of Wuthering Heights. Their relationship mirrors almost identically the love Catherine Earnshaw and Heathcliff once shared.
Like Heathcliff, Hareton has been degraded by the head of the household, in this case Heathcliff. Heathcliff has deliberately brought up Hareton as a rough, illiterate farm hand in order to revenge himself on Hindley, Hareton's father. Hindley had degraded Heathcliff to the point that Catherine couldn't think of marrying him. Hindley did this because he was angry and jealous that his father, when alive, had favored Heathcliff.
Like the older Catherine, Cathy is in many ways the dominant partner in the love relationship. She is more refined than Hareton. For instance, she knows how to read, and takes on instructing Hareton. She can be bossy and domineering, just as the older Catherine was, though apparently not to the same extremes. (This is a sticky point, as Nelly, who tells the story, had problems with the older Catherine and may have exaggerated her faults.)
Heathcliff had planned to get revenge on the people who hurt him. It would appease him for the way he was denied Catherine if he could break the hearts of Hindley's son (Hareton) and Linton's daughter (Cathy) the way his was broken. Yet when the time comes to destroy these two, Heathcliff has lost the will to do so. Therefore, they are free to marry.