Having an instinct to be pretty when you're socially considered male is a bitch. Is it about being pretty, wearing dresses, I'd say no, there's more to it, but it wouldn't be the same without that either.
Why do I feel so comfortable with my breasts (fatty gynecomastia), I'd be devastated to lose them, which makes no sense from a standpoint of being pretty, you don't need that to be pretty, is it about clothes? No, I'd be very happy to have men's clothes hang off those same breasts and drape over my waist as well, think of all the ways you can wear men's clothes that a man can't. Is it about sex with men? Well, men do that all the time, you don't have to be female to do that.
In one sense it's about getting your body to match your brain, but in another sense it's about getting society's expectations of you to match up with your inner self as well, the need to transition would exist even if it was entirely acceptable for men to be feminine, but it would probably be half as strong, or less.