In general, it is "social divide"
That is a major source of friction between Heathcliff and Catherine in Wuthering Heights. This tension is only relieved later in the tale.
Explanation:
Heathcliff was a common boy who was fostered and never had peculiar schooling or protocol while she was always favoring towards partnering someone classy and educated. Unluckily for her, when he arrived back to the Heights that's specifically what he became so her life became much tougher.