So. I think it's important to distinguish gender identity to gender roles.

Let's start with gender roles. Where do they come from?

I believe it's a mixture of nature and nurture. Women aren't as strong as men and are way less expandable from a survival standpoint. This means that in a society it would make sense to safeguard women, not sending them into wars, so a society can bounce back from loosing a war. Now depending on how society deals with this premise it can go one of two ways. In societies where strength is dominant men will be as well. In a society where wisdom is dominant woman can be, since their wisdom is less likely to be lost in war or battle.

Men becoming more dominant would then lead to women becoming more oppressed, since men want to hold onto their power. And what the easiest way to achieve that? Keep them both weak and stupid. Gender roles, roles one getts assigned by society based on sex, are one of the most important tools to keep men in power.

I'm somewhat transmed, so gender identity to me is just the identification with the other sex. What causes this? I have no idea. I have seen the hypothesis of shorter androgen receptors in trans women's brains floating around, but the studies to me seem inconclusive and I don't see a way to study this in an ethical way. So I'm open to any convincing evidence. For now, Im 80% positive it exists, but I don't know why.

Gender roles are forced on us every day. Following them stems Imo from a desire to fit in. Not many people would choose the outgroup, since it's a hassle and the immediate consequences of doing so are social. We're social animals after all.

Trans people follow the same principle, we're people after all, plus the added desire of wanting to be the other sex. If you want to be something, you'll try your best to become that, however impossible it may seem thus gender roles become a tool that helps achieving that. I don't think many people are conscious about why they do it, since they haven't thought about it a lot.