Styles

So I'm really confused. Why do styles leave and then come back?? Like the whole one-shoulder-thing. That was  really "In" when my parents were kids, and then it left, and then it came back, and is starting to become big again... I mean it wasn't even that long ago that it was in style... and then it went out of style.. and now its back... isn't it just kinda a waste of time??