there was a young woman from boobville ...

I think it could be because Micheal can't find Dean. Zack only found him by using humans as spys because Cas burnt that stuff onto their ribs so they're hidden. It could also be because Dean can't hear the angels when they talk in there normal voices. Or maybe Micheal is that powerful he needs permission from his vessel before he can even come to earth so he needs someone else to ask for him. Hmmm there's so many possible reasons isn't there...