I think I’ve mentioned before we are our habits. That includes things like considerate behavior. I can’t imagine what it would be like (to use an example FML gave) to live with a person who is constantly hostile. I’m sure it develops over time and both start to feed off each other. Come home to constant hostility/depression/whatnot and eventually one starts to associate the person with the feelings they have when they’re around them. I’m sure over time, it’s a feedback loop that’s hard to break (maybe impossible).
My ex is illustrative of this. At first, she was appreciative of what I had done for her and her son. Or rather, she acted that way. But then, old habits on her part started to come forth. As I learned later, these habits were part of why she divorced her ex. Basically, I would come home from work to a world of complaining. Her life was very good, far better than it would have been without me, but she couldn’t contain the complaining, It got to a point where she complained about excess wind, implying that it was my fault. Once she got back into her old habits (they started in childhood I found out later, complaining about her parents), there was no stopping her.
As for me, my blue pill self from time to time stood up to her, but mostly I withdrew into my very introverted space. Anyway, all of this provides one more datapoint as to why some people are just not good bets with respect to long term relationships