Women demand a man who will stand up for himself, even to her, even if it is illogical (to you) to do so sometimes. But, many nice guys labor under the misapprehension that the more they try to get along, the better the relationship will be.
You have to provide evidence that you're going to be a solid rock in her stream of unpredictable emotional episodes. I'll say it...many women lack stability in their own lives, so they'll seek it from you. The biggest mistakes you can make are placating and martyrdom, because they give women the feeling that you aren't willing to "take charge" and provide direction in their unstable lives. Giving in to her constantly tells her you are not in control.
Placating, or giving in despite objections, is the exact opposite of what women want. She doesn't want a guy who will roll over to her every whim. The best way of putting it is: she doesn't trust herself, and she wants you to be there to tell her when she's going wrong. Of course, most women would say that this is crap, and that they are more than capable of making decisions for themselves, but if that were true, they'd be seeking out the placaters instead of the headstrong men that they inevitably end up with instead of you. No, they don't want to make decisions, for the most part.
Before coming to this realization, one of my dates summed it up when I suggested that anything she wanted to do that evening was fine with me (stupid). She looked me in the eye and said, "You're too agreeable". Until she said that, I didn't know that could be a fault. What she was really telling me was that she was unhappy with the fact that I didn't demand control of the situation. Deep down, women don't want to be liberated, 21st century girls. They don't want to have to trust in their own ability to take control. It is your job to do that.
Worse yet is the martyr, who assumes that somehow, the girl is keeping a mental notepad of all the sacrifices he's making and someday, she'll reward him by realizing it, and perhaps as a consequence will treat him to a four-day marathon nookie session. Forget that, buddy. Every time you think you've added a point to her mental notepad in your favor, you've actually subtracted five. Every sacrifice you make will have the opposite effect that you intended. Instead of her seeing you as a great guy, she'll become more and more worried that she's stuck in a relationship with Charlie Brown.
In some respects, women are like children. The more you let children run with with their own impulses...the more that you let them get caught up in their own excesses, the less they will feel good about themselves, and the less they will respect you. The most well adjusted children are those who are given clearly defined limits and held within them.
I imagine that many will take issue with my comparison of women with children. Fair enough. Keep in mind that this does NOT apply to ALL women; just 99% of the ones that I've had experience with.
Think of it this way...in every battle of wills, someone has to win. You like to win, don't you? Then the winner might as well be you. And yes, I am suggesting that if you disagree, you provide at least some resistance on everything worth discussing, even if this is not the type of person you are. Why? Simply because it will be a constant reminder that she is with a man who is suitably aggressive, even preferring aggression in the face of resistance from her (if you'll stand up to her, the one you supposedly love most, she will believe you'll stand up to anyone else). Also, you'll probably get your way more often than if you simply give in and hope she'll return the favor.
Also, you need to remember that you can't just fake your way through this and then let your guard down. The instant you do, you will lose her interest.