HELP FROM GUYS!! Why do guys get turned on when I wear their clothes?!?

My best guy friend and I hang out allllll the time and the one night I stayed over at his house (we didn't do "anything".... Geez) He told me to wear his pjs and I asked him y and he said it's a big turn on but he won't explain how it is? So pretty much the question is.... Do guys find it attractive when girls wear their clothes and why?!? Thanksss :)