WHY DOES SEX CHANGE FEELINGS

Basically Iv started being fwb with this guy a few weeks ago who likes me but I never really liked him in that way until we had sex everything changed I now really like him and I don't know how it's like suddenly I went from just liking him as friend to near enough falling in love with me in like an hour, the sex wasn't anything amazing but my feelings instantly changed, I feel so infatuated by him.  WHY HAS SEX CHANGED MY FEELINGS HELP ME!!!