Okay, I've changed my mind for my personal canon after thinking some more about 4x03.
So I think I said in my last post that I thought it would be awesome if Bobby knew the Campbells, yes? Well, that would still be awesome, but here's what I thought of and would like EVEN MORE.
So, Mary made her deal on May 2nd, 1973, right? And the deal was that in 10 years, the YED would be able to stop by her house. So Mary probably thought that May 2nd, 1983, would be pretty significant, yeah?
Nothing much happened that day...except for, oh, yeah, she had Sam. So what if she thought Sam was, like, the YED or something? "I'll stop by your house in 10 years." As...her son? I think that's a logical thought process.
I can sort of see it in my head. They'd planned to have a C-section, on, like, May 18th or something. But her water broke early and out came Sam. It would be perfect if she actually had him at home, before the ambulance could get there. That all seems a little pat, but it would confirm Mary's worst suspicions, right?
So she has Sam, and he's her beautiful baby boy and she loves him and he can't be a demon, he just can't. She does what she can to test it, trying whatever exorcisms she knows and giving him a bath in holy water, and he seems to be just perfectly normal.
But the YED said he'd be stopping by. Demons don't just give up on their deals.
So she calls up someone that's made a name for himself as a demon expert, Bobby Singer. And Bobby drives down to Kansas. She tells him everything. Bobby does some more tests and lets her know that as far as he can tell, her son is perfectly normal.
But Bobby knows about Mary's deal. Bobby knows that Mary's worried that Sam isn't normal.
So when John Winchester comes knocking at his door whenever it is later, Bobby knows more about their history than John does. Bobby might even have suspicions about what really happened to Mary.
And then even later, when John hears whisperings that Sam isn't normal, that Sam has a demonic destiny...well, Bobby knows a bit about that, too.
So that's what the falling out was about. John found out what Bobby knew.
I'm posting this now because I'm sure it'll be totally AU after tonight's episode.