Heathcliff and Hareton

In the novel it comes about for a few reasons. Firstly, like splashen said Heathcliff did save Hareton when he was younger. Heathcliff also finds himself identifying with Hareton in the novel, as Hareton is in the same terrible position as Heathcliff was when he was young. Plus, he respects Hareton's physical/mental strength comapred to his own son, who he hates for being so feeble (something Heathcliff associates with the Lintons). In the novel it is also remarked upon by a few characters that Hareton resembles Cathy (moreso than Cathy Jnr), so that could be another reason as to why they form this bond. But, Heathcliff's feelings towards Hareton are very complex, and he does resent him for being Hindley's son. If I remember correctly there's a line in the novel were Heathcliff says "I would love him like a son if Hindley wasn't his father". That's not an exact quote though.

As for Hareton, well, Hindley was pretty awful to him .. throwing him off the staris and generally negcleting him completely. When Heathcliff retursn he pays attention to Hareton teaching him swear words and that his father is the devil etc. Although, Heathcliff does impose the life of a servant on Hareton like Hindley did him. Heathcliff probably paied more attention to Hareton than Hindley ever did. Plus, Hindley died when Hareton was quite young, and he spent most of his adolesence with Heathcliff as his guardian, so it was probably natural that he became attached to Heathcliff, he was the only father figure he ever really had.