When someone is tasked with feeling like they are doing everything (the weight of the invisible labor in a relationship or household) and they attempt to articulate it with their partner, he may see it as unimportant and inadvertently blow it off without thinking it through with statements like, "Why are you wasting time being frustrated about this?" He thinks it is too trivial to be important and has just dismissed something she has been annoyed with [insert issue] for weeks, months, even years. This is why, eventually, she just starts answering with "fine" and your relationship starts to sour more and more with the unresolved problems as they continue to build up. Then you wonder why, when we're tasked with doing 2x as much, we gain weight, we're exhausted, we sleep like shit, and we're never in the mood.
Instead of trying to see the deeper, root issue of 'she's tired of sacrificing HER time, which is just as equal as yours, to do things he should be doing his share of, he just hears, (for example) "I'm frustrated when you leave dirty dishes in the sink, even after I've asked you (nicely) to please not." See, she's the likely the one cleaning up and it would be nice if you didn't add to her load by tossing stuff into the sink and never simply putting it into the dishwasher. Might not seem like a big deal, but imagine following behind someone several hundred times and doing the extra step they didn't. It is inconsiderate and makes the other person feel like they are being used or taken for granted.
She doesn't want to be a nag. She doesn't want to feel like his mom. She doesn't want to be the one using up HER equally valuable time tidying up or having to ask that he do the things any situationally-aware person knows needs to be done. You pulled a heavy box she struggles to lift or move from a closet to get something and not put it back right away. You witness her trip over it a couple of times, and rudely suggest she put it away when you are the one who got it out in the first place. BS like that gets really old really quick. Why is it HER job to pick up after you? How is SHE a nag for asking you (multiple times) to do your share of something?