~~~Okay, I'm too sleep-deprived to be eloquent or concise, but here is my perspective anyway~~~
In first grader language: your brain feels that it is a girl, so everything in life that tells your brain otherwise makes your poor little brain feel out of step with its host. This makes your brain upset.
And I do mean everything- the big things, like seeing your image in the mirror; to all the little things you may not even consciously be aware of, like your friends repeatedly calling you "dude," or that obviously straight girl that looked you up and down the other day, or the sound of your voice, or maybe even the hormones coursing through you. You may not consciously notice all of that... or any of it, but your brain does. Some of those things are physical aspects of your body; most of them, from clothing, to social roles, to how you interrelate with others, are nothing more than constructs of society. But they all work together to tell you where your station is.
The more of this wrong feedback you can eliminate, the more comfortable your brain will be, and thus the less angst ridden and miserable you will be (your brain is you, by the way- not your foot or hip.)
Why can't you just express yourself more femininely and be happy, maybe take up knitting or something? What exactly will knitting do to change the giant cloud of gender feedback that surrounds you? Nothing that's what- besides maybe adding one weakly feminine variable to the mix. This is why you probably find dressing as a woman offers you more relief than, say baking. Clothing and physical expression change many, many of the variables in that cloud, many of which (like the way you look) happen to be ones that are relentless in reminding you (your brain) that your host (your body) is not the same gender as you.
So the next question is why is gender so important to your brain? Is it not just some superficial external expression? No, it isn't- far from it, in fact. Name one other attribute that that is more informative about a person than their gender. "Black man" "poverty-stricken woman" "dude from Tennessee" Notice which word is the subject noun in those phrases.
Take the following sentence: A straight black woman from New Orleans with a PhD from MIT now lives in Holland working as an astrophysicist. If you were meeting this person for the first time and this was all you knew of them, which part of that description tells you the most about them and that informs how you expect they will act, look, and how you should plan to treat them? That she's black? I hope not. Again, notice the subject of the sentence.
Gender doesn't just matter a lot, it matters more than anything else. It affects every little nook and cranny in your life and you can't escape it.
