What issue was a major source of friction between Heathcliff and Catherine in Wuthering Heights?

I think it is safe to say that Catherine's marriage to Linton was a source of friction for Heathcliff! And it is not simply because he was dumped. Catherine's marriage amounted to a betrayal of some fundamental truth Heathcliff shared with Catherine. Heathcliff and Catherine connected in a primal way; what they shared was beyond physical attraction, or simple friendship. When Catherine says to Nelly, "I am Heathcliff!" she means it in a literal sense -- so attuned is she to him, she feels she understands him utterly. Yet the scene where she says this is also the same scene where she confides to Nelly that she could never marry Heathcliff -- that he is too poor. For Heathcliff, who is listening, this is too much to bear. Catherine's deep connection to Heathcliff cannot outweigh her imagined mortification at being the wife of someone of such inferior social standing. We readers can perhaps forgive Catherine for thinking practically about marriage, given the dependent status of women in her time; but in a way Catherine is also betraying herself, her own personal agency, her own love of wildness and freedom. In the world Heathcliff and Catherine share, it should be impossible for her to marry Linton, but nevertheless it happens.