He knew he shouldn't have done it. He knew it would probably ruin things between you two. You'd known each other since you were kids. But he couldn't help himself. As much as he didn't want to admit it, he was in love with you. And that's what led to Sam sleeping with you.

It had been a heat of the moment type thing. You'd both been a bit emotional. A bit tipsy. A bit needy. And Dean had been gone, out with some chick he picked up from a bar... Despite you being his best friend... he made a move, and it escalated.

Now, this was months ago. The tension between you two was strained and awkward. Dean noticed. He didn't know what it was, but it was missing him off, constantly being the mediator.

So, one day at a diner, Dean muttered about having to go to the bathroom, saying he'd be in there a while due to a 'stomach ache', leaving you and Sam alone, sitting across from each other, at the booths...