Simple. Being dressed up as a girl by your sister when a toddler and enjoying it. In later years around puberty revisiting that wardrobe and trying on her dresses recreates that enjoyment. Masturbation while cross dressed seals the deal.
What Causes Transvestism?
A lot of men are usually fascinated by women's clothing and feeling sexy; while others may have extremely inner-personal relationships with their Mothers. It's a feeling within each person that comes out that has to be expressed from within. It's their own feeling that they are expressing when they perform this type of social identity.