Part of the reason is people's ignorance of the other side's socialization. We were not magically created as adults, but began life as babies, who grew up into children, then to teenagers, and finally adulthood after many years. During our formative years, we learn things, culturally, about how a man or woman is supposed to act. These lessons quickly become subconscious. We act on them without even realizing it.
As an example of this, emotional expression. Men in American society are taught that it's unmanly to show any strong emotion other than anger. A man can be weakly happy or sad about something, but a man who bursts into tears expects to be derided as pathetic for doing so. On the other side of the gender divide, an American woman is expected to show more happiness than her own feelings have. She's expected to be majorly happy in minorly happy circumstances, mildly happy in neutral circumstances, and neutral in mildly sad circumstances. Also, she's not supposed to show anger, no matter how much she feels. Typically, she sits on such feelings until they explode. Both of these lead to major misunderstandings. She finds his apparent indifference infuriating, and he finds her abrupt outbursts irrational. (Because she's not just angry, she's burning inside with the fury of a thousand suns, and if she were merely angry, she would have kept silent.)
A common complain from both genders is a lack of observational skills. You changed your clothes, or your haircut, and your significant other doesn't even notice! I usually hear this from women, though a poll I read suggest that many men feel gripy about this too. Perhaps you worry that if your partner doesn't notice that big a change, then maybe they wouldn't notice if you were replaced with a completely different man/woman (insert as appropriate) altogether! Though I think this is normal. People do tend to be change blind if it happens behind their back. If a man doesn't watch his girlfriend's hair get cut, then he'll run into her later, and stop his noticing at "my girlfriend is here." That's good enough for him, but not for her. Apparently, the other way around works too.
I have a report here, from Japan, about the top ten things that Japanese women are confused about their boyfriends or husbands, and 9 out of 10 of them apply to American relationships too.
1. He was desperate to date you, but once you started dating he no longer caresI think this is a "thrill of the chase" thing. When he was single, acquiring a relationship was a major goal. Now that he has a girlfriend, doing things to further the relationship don't occur to him.
2. He thinks he’s really doing well if his girlfriend can cook a stewI think this is Japanese-cultural specific, because it means nothing to me. Stew? It seems to be saying that the men in this relationship are impressed with things that the women consider trivial. (He's impressed because he can't cook at all, and she's not impressed because she's made this a gazillion times while single, what's the big deal?)
[there appear to be some substantial differences in opinion between the sexes as to how easy it is to prepare Japanese cuisine based on stewing, or "nimono"]
3. He thinks it’s all over once you get marriedIn both America and Japan, women expect their standard of living to go up when they get married, as they will share in their husband's resources, get higher status in society as a household leader, and present a unified face to society, and she can now have socially respectable children if she desires.
The men in both societies, however, are expecting their standard of living to go down. They expect that their now wives will take the relationship for granted and stop trying to impress them. They expect to be pressured to buy expensive things to improve the wife's social status. Many of the things that affect him now are now out of his control. If his wife wants children, and he doesn't, he will be pressured to have children anyway, and society will mock him as selfish for resisting. If his wife doesn't want children but he does, he will be expected to again cooperate with the wife's wishes and ignore his own. (Though society does encourage women to want children, and pester them if they don't. A husband is not permitted to be the source of this pressure.)
4. Even though he has a girlfriend he still goes on group dates (goukon) for funI'd want to know how serious going for a "group date" is in Japan. Assuming the worst, that it is a romantic kind of thing, I suppose one could see it innocently as enjoying the attention (but coming home to the girlfriend every day, thus choosing her over the other girls), and at worst as being a total womanizer who wants a massive harem.
5. Whenever he’s with his friends he starts talking toughThis is a socialization thing in both our societies. He's expected to show a tough and macho face to his friends.
6. He thinks he can see his girlfriend any time he likes so she becomes a low priorityDid you know that neurological studies show that considering the opposite sex makes the part of our brain associated with tool use light up, and the part associated with dealing with people remain quiet? (Shame on him anyway.)
7. He tries to impress with his manliness by eating large helpings and hot dishesThis is a socialization thing again. Men are expected to extremity any factor that distinguishes them from women, hence what started with eating more because he's larger (and requires more food to maintain his body) turned into eating even more than that ... because he's male. The spicy thing is because men are expected to be stoic in the face of pain, to the point of inviting it to demonstrate his might. (I'm not afraid of spicy! Rrrarrrgh! ow ow ow ow make it stop)
8. He says he doesn’t see the point in marrying so he won’t marryHe thinks his quality of life will go down, and doesn't fully grasp how his girlfriend's quality of life would go up.
9. He won’t ask people about things he doesn’t know as he doesn’t want to be indebted to themA man in American society is expected to be independent, and to know things. Not knowing things (or at least admitting as such) is being dependent on other people. For an American man to admit that he doesn't know something will make him feel like he's being childish. Apparently a different, but related, dynamic appears with Japanese men.
10. He ignores his girlfriend and talks solely about himselfHe knows himself better than he knows his girlfriend. Been together longer, you see. This is probably a bad habit left over from single days when he was only expected to look out for himself.