I believe that social pressure can cause transgenderism.
In general, men who embrace traditionally feminine qualities are shunned. The same goes for women embracing traditionally masculine qualities. I don't think there is anything wrong with a flamboyant man or a tough-skinned woman, but there is, to some degree, ostracism of people like this, especially in crucial developmental stages (middle/high school age).
As a result, people with these "unacceptable" traits feel like there is something inherently wrong with them. Some come to the conclusion that their natural gender is wrong, so they change it.
This is just food for thought, really. It obviously doesn't happen like this for every transgender, but I do know somebody personally who I believe fell prey to a situation like this. I obviously can't know what they are thinking. I could be wrong. Who knows.