Mating in mammals has a basic asymmetry – females must invest more in each child than males. This can lead to an equilibrium where males focus on impressing and having sex with as many females as possible, while females do most of the child-rearing and choose impressive males.
Since human kids require extra child-rearing, human foragers developed pair-bonding, wherein for a few years a male gave substantial resource support to help raising a kid in trade for credible signs that the kid was his. Farmers strengthened such bonds into “marriage” — while both lived, the man gave resources sufficient to raise kids, and the woman only had sex with him. Such strong pair-bonds were held together not only by threats of social punishment, but also by strong feelings of attachment.
Such bonds can break, however. And because they are asymmetric, their betrayal is also asymmetric. Women betray bonds more by temporarily having fertile sex with other men, while men betray bonds more by directing resources more permanently to other women. So when farmer husbands and wives watch for signs of betrayal, they watch for different things. Husbands watch wives more for signs of a temporary inclination toward short-term mating with other men, while wives watch husbands more for signs of an inclination to shift toward a long-term resource-giving bond with other women. (Of course they both watch for both sorts of inclinations; the issue is emphasis.)
This asymmetric watching for signs of betrayal produces asymmetric pressures on appearances. While a man can be more straight-forward and honest with himself and others about his inclinations toward short-term sex, he should be more careful with the signs he shows about his inclinations toward long term attachments with women. Similarly, while a woman can be more straight-forward and honest with herself and others about her inclinations toward long-term attachments with men, she should be more careful with the signs she shows about her inclinations toward short term sex with men.
For both men and women, carelessly strong signs of an inclination toward betrayal could needlessly break their marriage. Of course it may sometimes be in one’s interest to show weak signs of such an inclination, as a threat to induce better terms of trade in the relation. But such brinksmanship should be done very carefully.
Men and women may have evolved, either genetically or culturally, to adapt to these pressures on their appearances. If so, then we should expect men to be more self-aware, transparent, and simple regarding their feelings about short-term sexual attractions, while women have more complex, layered, and opaque feelings on this subject. In contrast, women should be more more self-aware, transparent, and simple regarding their feelings about long-term pair-bonding, while men have more complex, layered, and opaque feelings on this subject. By being more opaque on sensitive subjects, we can keep ourselves from giving off clear signals of an inclination to betray.
Standard crude stereotypes of gender differences roughly fit these predictions! That is, when the subject is one’s immediate lust and sexual attraction to others, by reputation men are more straight-forward and transparent, while women are more complex and opaque, even to themselves. But when the subject is one’s inclination toward and feelings about long-term attachments, by reputation women are more self-aware and men are more complex and opaque, even to themselves.
So let’s sum up. Why don’t men express their “feelings”? (At least about “love” – they easily express “feelings” about sex.) And why don’t women know when they are “horny”? Perhaps because such knowledge is dangerous – if you know it, then others may learn what you know from you. Which might destroy your marriage. So our feelings may be most opaque to us when we need them to be opaque to others. Homo hypocritus mates.
Added 10a: Similar incentives apply in the gradual creation of a long-term bond. He slowly becomes more inclined to devote resources to her over a long term, while she slowly becomes more inclined to become sexually exclusive with him. Neither side should too easily give all they have to offer before the other side has given all it has to offer. Opaque feelings help to manage such a slow matched escalation in feelings.
This whole story requires that given ambiguous signals people tend to assume the best, rather than assume the worst. Seems to apply to people, though I’m not sure why.
Added 1Aug: As I commented, “husbands having outside sex, and women breaking off the long term relation, are both weaker forms of betrayal than vice versa. As a weaker form of betrayal, people feel more free to do them.”