ok - a personal answer and then a general theorem
For my own internal processing, it's not so much wanting to be, as not wanting to be. I can think of almost any trait, physical or attitude wise, that is characteristically "male" and the idea that I might be that is repulsive to me.
I know that I'm playing into, maybe a victim of, stereotypical thinking in some of these, but it's a visceral reaction...
I want to be soft, not hard
i want to be weak, not strong
I want to be gentle, not aggressive
I want to be submissive, not dominant
I want to be small and petite, not big and broad
I want to be curvy, not angular....
I could go on and on. when I walk through the store or the mall and i look at the men and women around me I can't even get past this line of thought to get down to what I'm sexually attracted to - I just see a guy of any age, in a suit or in overalls, looking dapper or dirty...and my heart says "I don't want to look like that!!"
and I see a woman, of any age and almost any shape (I confess that the more masculine women kind of trip the breakers a bit) and whether she's a bare-midrift teenybopper or a soccer mom or a graceful older lady I...resonate...with that persons appearance and conduct and mannerisms and all the rest.
In fact, when I force myself through all the fog and really look hard at my sexuality, it's so weird that I can't remember the last time I look at a good looking woman and thought to myself "man i'd like to ____ her!" even though I recognize, on an intellectual level, that she is sexually desirable.
On the other hand,i can imagine myself easily as the sort of woman who is quite promiscuous and insatiable when it comes to sex with guys even though when I look at them (now) I don't really see what there is to be attracted to (not to say that I can't call a good looking man a good looking man...for a man)
I here my sisters sometimes, in other forums, talking about hooking up with this guy or that and I think "damn that sounds like a lot of fun!" but yet I don't find myself looking at guys normally, when I'm not dwelling on my sexuality, in that context.
anyway, that's a pretty huge tangent I guess but what I'm getting at is that all these feelings are instinctive, visceral. When Nero asks "why wouldn't anyone want to be a guy?" the very things he's seeing as advantages are in fact the very things which repel me.
Now, maybe I'm presuming too much to extrapolate these things to a general hypothesis but here goes:
I would suggest that the reason we want to be women is because by nature's designs, men and women respond to and crave different stimuli, different interactions with the world around us.
The very things that are seen, in the abstract, as "male advantage" are the things that are stimuli that the "normal" man responds to. And having female minds, they don't impress us at all.
And the same applies in reverse - the things our female mind craves as stimuli can't be found in the male world, but those things are repulsive to those of you born female in form but male in your thinking.
you crave the very stimuli that I and my sisters recoil from.
we can no more understand the "why?" for each other than a cis person could understand either of us. We just have the experiences which equip us to accept the validity of each other's respective difficulty.