Men and women have certain cycles that most follow. Women are described as "waves" whereas men are described as "rubber bands". Not everyone is like this, but most people are.
The wave means that women are sometimes happy, and sometimes sad (we all knew that already). But did you know that in order to be truly happy, they have to feel the sad feelings? In the long term, to repress angry or sad feelings is to repress all feelings, even happy ones. Guys, never ever discount a women's feelings. That is who is she is, and to say she's being unreasonable is to discredit her being (more on this in a bit).
When women are at the top of the wave, they happily give love and nurture. But when they are on the downward movement of the wave, that's when they need caring the most. As a man, the best thing you can do is sit down and listen. And when she's relieved stress through sharing or ranting, she will eventually be on the upswing of her wave. But if you tell her "be reasonable" or "bitch you crazy!" she will be angry at you. Also, you're not allowing her to hit the bottom of her wave, which is the only way you can go up. It is a necessary catharsis. You see this in abusive relationships where the woman just becomes lifeless and stops caring.
Guys, on the other hand, are like rubber bands. When a man has been single for awhile, he feels the need to dedicate himself to someone. It makes him happy to do so. This is what makes a man go out of his way to impress a lady (grooming, dating, etc.) But a man also has a cycle. When he has been intimate for some time, he misses his independence. I'm not saying he wants to date other women. It just means he misses hanging out with his guys or going to the gym or just playing video games alone. This is his time to relax and find his identity again. It can be anywhere from a few hours to a few days. But the guy will come back to his lady and pretend that nothing ever happened and everything's cool. Once the rubber band is stretched, it will come back. Ladies, you must let your man have his freedom. If you try to repress him, you will either have an argument or he will repress his masculinity (just as an abusive guy makes a girl repress her feelings). We see it in relationships where the guy is whipped.
Do you see the problem with the differences? Why do you think some relationships hit a hard point after the honeymoon period is over? The guy starts to give less attention than he had before, and the girl isn't as lovey dovey as before, because she's on her downward wave and needs the guy's support more than before.
But it's okay! There are compromises! Knowledge is power, and it can help prevent many arguments. If a man knows how much women appreciate listening, he can feel that he is succeeding in the relationship instead of trying to figure out why his wife/gf is unreasonable (meaning he thinks he's failing somehow). If a woman knows why guys need their personal space, they won't feel abandoned. The guy needs to recharge his batteries and will come back. In this one case I think it's easier to be a girl than a guy. If a girl is having a bad day and the guy goes golfing, he's effed. If a guy is having a bad day, and the girl goes shopping, both sides will probably be happy by evening.
Men: if the girl looks distraught, you should ask her what's wrong and listen to her. If she says nothing is wrong but looks pissed, something bad happened. And you should let her know how much you care about her and that you will there to listen to her.
Women: if the guy looks pissed at something, ask him what's wrong. If he says nothing, give him his space. Don't baby him, it might annoy him more. He will get over it on his down.
*disclaimer* this is for most people, probably not applicable to manly girls or girly men