Why people become she male?

They're not typically referred to as "she-males." In fact, that's a little derogatory. Gender reassignment takes months, even years, to accomplish. It starts with intense counseling to determine individual needs and progresses to hormone therapy and finally reassigment surgery in which surgical procedures are performed to create female genitalia.

The process is much more involved that outlined here, but that's the overview.

The same way women do, by becoming a transvestite.