Him: "Women are crazy."
Her: "Men are stupid."
Women have mood swings. Men try to solve women's mood swings because it really stresses a man out when his mood changes. Men prefer to be in the same mood all the time. Women like to have mood swings. It makes them feel better.
So when a woman have mood swings, she feels alive. But a man sees mood swings, he thinks the woman has a problem and tries to solve it. But to a woman, having mood swings is how she solves it.
The reality is you should just let her have her mood swings and accept it. Don't bother analyzing her mood swings like there was a reasonable explanation. Her mood swings is a good thing. Your ability to handle her mood swings will give her confidence in you. It is a challenge and opportunity to bring the relationship closer based on your reaction. Do not whine, complain, question or attempt to solve her mood swings. Do accept, support and allow her to be moody.