There are a lot of people who get mad at you from the moment they're attracted or interested in you due to their control issues, so they're mean towards you almost before the relationship even starts because of their own feelings of vulnerability. (If they really loved you -- or were even capable of love, they would care about your feelings as well).
They feel
want, not love. So they're unconcerned about your feelings at all, and any 'niceness' is sporadic, manipulative and disingenuous.
I'm not sure all of that describes J.R., as the chase -- the conquering -- of the other person seemed to be his main goal. So he was only interested in Sue Ellen whenever she showed signs of leaving, tiring of his neglect and abuse. Once he had her back, his disinterest re-occurred. Repeatedly.
Sue Ellen, on the other hand, was raised to catch A Rich Man, status & money being her agenda. So it's hard to feel much sympathy for her.
J.R. & Sue Ellen were a rather Old Testament biblical couple: the unloving Everyman and the unloving Everywoman. (Only, she could use her gender role to play "the good one," which is why a lot of narcissistic, non-introspective women adored the character, just as Princess Diana fans adored and idolized
her, excusing all of her own behavior).
Their bond, as it were, was by definition sick and mutually exploitative. But Sue Ellen had her own problems with her own "the chase" patterns: whenever she was with a nice man -- even a rich one -- she'd get bored; returning to the rush she felt within the cat-and-mouse dynamic she had with her husband.
She preferred The Bad Boy and the frustrations that went with it.
So they
really were deceptively alike. Larry once pointed out that "JR had a need to abuse; Sue Ellen had the need to be abused." And once Sue Ellen finally tired of it all, she just flipped the sadomasochistic nature of the relationship back on JR, and then he seemed to succumb. So the game went on.
The healthier thing Sue Ellen could have done was simply leave. But she wasn't willing to do that until Lorimar-Telepictures no longer wanted to pay her.