I think the point is that it is a doomed love story, and we read about the repercussions of these two not being able to be together. Both of these characters are absolutely unbearable, but that’s the point—hurt people hurt people. These two lash out at others because they are in pain, because they can’t be with the person they love. If they had been able to be together, they would have changed each other for the better. Heathcliff would have mellowed out Cathy’s brattiness and Cathy would have been able to subdue Heathcliff’s sadistic streak. In my opinion, the real tragedy of the book is not how the two lost each other, but the loss of the potential they would have had together. What is truly heartbreaking about the book is that the two of them could have been good people, but pain hardens them and they choose to lash out at the world. They are then juxtaposed by the younger generation, who choose kindness and growth, which demonstrates one of the themes of the book, which is that life and pain and suffering will happen to you, but it’s up to you to decide how you react to it.