I can remember little bits and pieces from my past, but no real "why".
The first time i tried on panties, (also the first exclusively female clothing item ever), i wasn't aroused. I was literally too young. i was simply curious about what it felt like. I was instantly jelous of the incredibly soft texture- but after a moment, i changed back into boy underwear and carried on with my day.
A few years later, i was truly pubescent, and sexual thoughts began to play a role. As of now, i like to fantasize about being vemale, but I lnow I do not wish to transition. It can be puzzling, though, when the "pink fog" descends and all I want to do is dress.