And MAYBE...

Another thought about destiny:

Destiny doesn't have to be absolute.

For example, destiny might dictate whether I'm going to be a mega-bestseller or a moderately successful author whose greater value is in helping other authors on their paths to success.

But destiny probably does not dictate whether or not I remembered to pick up the dry cleaning on my way home today (I didn't).

And probably, "destiny" cannot be determined for certain in most circumstances. So let's look at the show again. It was destiny for Mary to make that deal with Yellow-Eyes, which Castiel knows because he tried to "change" it and it couldn't be changed. But that doesn't mean that every single decision made and action taken is destiny and PREdestined, and they can't know in advance which things are and which aren't.

One other thought...

Number One and I were talking about Mary's story, and discussing when it was that Eric Kripke and/or the writers figured out what it was. One of the things I love about writing, and being a seat-of-the-pants writer, is when I figure out a fact about my characters or events and then go back to insert the clues that lead up to that fact, only to find that they are already there. It always impresses me when that happens. :)

So...

Did Eric Kripke know, when he first put Mary on the page, that the circumstances of her death were not a random occurrence? That it was actually Mary that caused YED to pick Sam? Did he know that YED's endgame would extend beyond his death?

Or did they decide, as season 2 neared and they were connecting Mary's death/Sam's selection to the current chosen-kids battle, that she had greater involvement? And if they decided then, was it just that she recognized him and they'd figure out why later? Or have they been holding on to all that for all this time?

As I type that, I remember them saying that a lot of the Mary stuff had to be set aside for season 4 because of the strike, so they probably knew, when they scripted the "You!" moment, why she recognized him. It will still be interesting to hear from Eric when that developed, if anyone ever asks him.