Many women can't but it's not because they were born that way. It's the matter of years of influence from others...
Interestingly enough, it is among primitive people, in societies where there are no sexual taboos or strong pressure to conform to some artificial standards that both women and men enjoy sex the most (Malinowski's studies come to mind, as well as some anecdotal evidence).
I have my own theories, and they boil down to the way the person grew up.
I think that most people are subconsciously petrified of what others will think of them, and that fear is omnipresent - in every situation, and all the time... and it paint's that person's entire life. It is sad to see it (it makes joy of life impossible to experience), and I am afraid that some 90% if not 99% of people live like that. Most, if not all things in life, we can still do even when we are afraid of "what would others think"; sex is one thing where we cannot cheat ourselves - either we feel perfectly good about ourselves and what we are doing, or we don't. If we don't - sex doesn't work.
I've never met a person who had only sexual problems - they all had some deeper issues, but those were ignored (as those other issues were socially acceptable, even desirable)
But getting to the bottom of it all, is not easy nor simple. Most psychologists today get into that profession to learn how to mask their own problems, and have no ability to help. Most information today is also skewed in some way. This all means that becoming a happier person is hard work and requires years of persistence and separating the wheat from the chaff. No short cuts will do, and there is no easy fix. Most of "quick" answers are just "emperor's new clothes" type of thing. Everyone will yell "yeah, me too" when the fix never worked for them; but they will never admit it.
It is good to get familiar with female anatomy (as well as male) - things like G-spot, clitoris, and so on help one understand that it's all there for a reason. Those organs and tissues wouldn't be there if we were not meant to enjoy sex. Beware that most "new research" was written by sexually frustrated who are looking for a way to explain that "it's not me"... Try to recognize it; wrong suggestions/beliefs can hurt more then open abuse. E.g. suggestion that some women just can't have an cllmax, or that some women have a clitoral cllmax while others have vaginal (as if there are two types of women - just feeds into the problem, doesn't solve it). All women are the same, and can have both cllmaxs.
Women which were supported in being "women" as they grew up, and who didn't feel guilty about themselves and were not criticized can hardly believe that there are women out there who can't enjoy sex. Even subliminal suggestions of todays society are unhealthy in that respect - just look at all the men in decision making positions as if women are incapable of that. In native indian tribes it was mostly women who decided who the next chief would be because they knew much better what someone was like as he was growing up... I think that most women will take some sort of a negative clue from today's male dominated world. And to make it worse - only hawkish women will succeed in such a world (and be even more sexually frustrated).
Couple of things I've noticed: women who have vaginal cllmax (g-spot cllmax) have it more easily with a partner, and almost exclusively "from behind" (because of anatomy). Clitoral cllmax seems almost impossible to achieve during intercourse (anatomy again), and almost every woman I met could achieve it while masturbating.
I also noticed that cllmax is harder to achieve when the person is feeling under pressure, and most people are like that. Those are usually the types who are very nice, very caring and so on - they don't stop thinking about other's people's needs even when they should. And it is impossible to think of your own needs and those of others at the same time.
Also, the typical modern society attitude towards sex, which is very materialistic, doesn't help at all. All we hear about sex is - how many times is "normal", what is normal to do, what is despicable, does the size matter and how much, or is it girth that's more important, "1000 ways to improve your sex life", "millions of sex secrets", "100s of new positions which will make you a sexual expert", what's your bra size, how to remove that cellulite, is there too much pubic hair, is there too little of it,
Body Odor , mouth rinse, long legs, short legs, hairy legs, too skinny, too fat...... You get the idea. There is no place for "quality" in the world today. As in - "This is what I LIKE" or "I want this".
If you know what you like in sex, then you're on your way to happiness. If not - try to work on making yourself happy on your own in every aspect of your life, and not just in sex (have you ever enjoyed a walk on your own? Watched a film on your own just because you wanted to do it? Made your own meal?). And when you do that, make sure no one criticizes you for doing it, or rather - for not doing something else that in their view was more important. Practice doing things just for yourself, without feeling guilty and planning how to explain the fact that you have needs also (make sure things are not left unfinished though - let others know that you need their help in order to rest or do something else a little). This would be the first and most important thing towards being happier in every respect including sex.