Do we always know how much we (girls) are into someone? For a guy it's simple, eirther he's there 100% or not at all. Guys won't waste time (and money) on someone they don't really dig. For girls, it can be confusing and arbitrary and I think it's because we settle. Guys don't settle. Really. Most girls can do better than the partner they have, but we're too sentimental and sensitive to leave them out cold, and we feel guilty when we ask ourselves, "Is he the best I can do??" Guys ask themselves that all the time.
We're also considerate out of compassion, that's why we wonder why losers we've turned down a hundred times keep coming back all hopeful, as if there's a psychological barrier and we're just not getting through. We also wonder why some guys assume we like them even though we just don't want to hurt their feelings, and by the time they've told all their pals how we're head over heels for them, we know it's too late.
You'd think He's Just Not That Into You is a condescending movie telling women who "don't get it" to pick up the right signals and read them properly. But actually part of its message is empowering: learn to tell who is worthy of your time and energy, and chuck those who don't recognize your value-- even if it's your husband.