What causes someone to be transgender (trans)?
We aren't absolutely sure why someone's gender may be different than the sex they were assigned at birth. It's probably a variety of factors combined.
We do understand that being transgender isn't something that can be forced on someone or something that someone can choose to be. It isn't something that happens because of childhood experiences, and it isn't a personal choice. It's inherent, in the same way as sexuality or eye colour.