Brains are such funny things. You learn something one way, and you cling to it because your brain has formatted itself like that. That's why people (say, who live or work together) have fights about stupid shit like where dishes get stacked or how a shelf is arranged: one person's logical arrangement will just piss the shit out of somebody else because the pictures they have in their heads of the way things should be differ. On a much larger level, of course, there's all sorts of things like racism and genderbashing that happen because we're so damn angry when our heads and the outside world don't match up exactly. The more strongly you feel about your interior picture, the more likely you seem to be to protest when that picture is disrupted. Gender is probably one of the things that most people feel most secure in, and so it's one of the things that seem to raise a lot of resentment and assholery when it's unclear or when it changes.
Some of us get the brunt of that every day, and some of us don't. We all experience gender, gender expression, gender policing, in different ways. We all foist our own gender expectations onto other people from time to time. We all have brains, and all of our brains were raised in a society, a culture, a world where we were told that certain gender rules applied. It can be hard to break those rules; it can be hard to accept that those rules can and should be broken. But once you begin to see them it's like a whole other world, one where artifice and reality begin to collide and smash up against each other and reveal entirely new conceptions of self. Think of it as exciting.
Sometimes, I can feel the gender edifice in my mind get shaky. When I'm thinking really clearly, when I'm talking to somebody and the gender ideas keep slipping into each other and I realize how transitory and constructed they are, I can feel it quake inside of me and for a brief moment I get a glimpse of something beyond what I normally perceive, gender-wise. I'm trying right now to make those moments happen as often as possible, because they always slip away from me and I'm not sure I'm learning enough from them.