The paid spouse is busy all the time. Like, so busy that he almost never makes it home for dinner, works on Sundays, and says that he doesn't have time to buy presents for anniversaries slash valentine's day slash anything else. He has a hobby he loves, but has had to cut way back on. He is resentful of that.
He wants to be at home more, but he has to work. He has a (large) family to support. He gets defensive when his partner points out how little he's home. He lets her know that he wants things too. He feels a large sense of responsibility and wants a little appreciation.
The stay at home spouse is busy too. She starts backing out of all her other commitments because something has to give - the partner she needs is rarely home, after all. She takes on more at home, she gives up the volunteer work she used to enjoy, she digs deeper and deeper and she's sure that one day this insanity will stop. She has sort of forgotten what hobbies she used to love. She resents that her spouse would rather be somewhere else when he could be at home, but she doesn't want to force the issue. She wants him to get what he wants - just not at her expense. So far, they haven't figured out a way to make that happen.
She wants a little appreciation too. She worries someday he will regret missing so much.
He gets frustrated when she gets upset about him being gone so much. He points out he has to work. She agrees. She is appreciative, but she doesn't want just a paycheck - she wants a partner.
He's not a slacker. He helps when he's home; he rarely complains when she goes out with friends. She's stopped going out as much; she misses him. She feels hurt when he doesn't give up going out to spend time with HER. She knows she is lucky in the spouse department and wonders if he feels the same...
She feels lonely and he feels stretched too thin. She takes the joy out of what he loves by hating it so much. She feels bad about it, but it's nothing new. She doesn't know what to do. He probably doesn't think about it at all. He probably thinks things are fine - or he hasn't noticed the sadness in her eyes. He leaves all the decisions about the kids/house/life -- all the worrying -- to her. Things start to fall through the cracks.
Why doesn't anyone ever tell you how hard it is to do this and keep yourselves together? It's so difficult to ask for what you need, to try to squash resentment, and not keep score.
Those fights are never about the trash. Anger is a secondary emotion, fear is the primary emotion. She is afraid that he doesn't value her contributions. She wants to know she is not doing it all on her own. He is afraid that he will never make her happy, that no matter what he does, it will never be enough.
~ Babyproofing Your Marriage