How can someone become a transsexual?

Nobody becomes transsexual. It's not contagious, and it's not something you choose. Instead, it is something to get rid of. You come to understand that the problems that have hounded you since childhood are not things you did nor anything mommy or daddy did either. Transsexualism is a disorder of the body: the psyche is built around a specific gender, in this case the gender of the psyche does not match the physical gender of the body.

A transsexual person spends a large portion of their life figuring out why their own body is alien to them, and if it is bad enough OR they are in a situation where it is not harming others, they go to a surgeon who aligns the body with the mind. There is a small percentage that regret this but it is very low, and most who regret it are really transgendered rather than suffering transsexualism. Because of the transition protocols, it can drive a transsexual person mad waiting to get ok'd for the surgery.