Overpopulation;
too much art and interactive media flooding their brains since infancy (anime and video games fetishises a lot of things and most kids even in western societies develop feeling and sexual arousal towards characters. Japanese characters are made to be seen as "real people);
they're perfectionists. been a problem since their middle ages. they are easily disgusted by normal things;
they're too intellectual and repress emotion too often. they're flooded with ideas, concepts, limits, requirements, goals, etc., since birth. They never get to just relax and enjoy their nature and bodies in a natural way;
and so on and on... It's not a single cause, I doubt it.