Masturbation while wearing women's clothing develops into a repeated script. The clothing adds a visual and tactile aspect to the masturbatory experience that make it more intense, pleasurable, and unforgettable. As humans, we tend to seek out and repeat pleasurable experiences. Because our boy's hormones are raging at puberty, his intensely pleasurable masturbatory cross dressing tends to get repeated a lot. Unfortunately, America isn't Samoa.
Masturbation is almost as big a taboo here as wearing the opposite sex's clothes.
Besides touching the wrong body parts, he is also wearing the wrong uniform while doing so. The social felony is thus compounded, and, being a felon, our young cross dresser gets a double dose of the guilt and isolation that society reserves for those who violate its taboos. So, that's the sexuality of cross dressing. Well, lots of women will tell you that males have only one thing in mind. While this is probably an exaggeration, I think most males would admit that sexual desires do take up a great deal of space in their private thoughts.