Based on Season 2, Episode 9 "Croatoan"

Dean knew that all of reason was against him – that any hunter in his right mind would gank Sam in a split second if they knew he was infected, and yet there was no way Dean could have pulled that trigger. It would take a stronger man than him – and even then they would have had to get past Dean who was always on his best form when protecting Sammy.

And Sam was asking him why Dean wasn't killing him – where his logic was. But the truth had no logic at all.

I can't kill you Sammy, Dean wanted to say, wanted to scream at the top of his lungs. I can't kill you because you're all I've got left in this crazy world. And if it wasn't for you, I would have stopped long ago; given up the ghost, for use of a bad metaphor.

You are my drive Sammy, my inspiration. You are the one thing that keeps my feet on the ground. All these innocents we save? I do that for you; for what you could have been in another life. And don't give me those puppy dog eyes Sam, 'cause I'm not done yet.

Without you in this world Sammy, there is no point in the job I do. You are my point – my bloody reason for existing. From the second Dad thrust you into my arms as a baby I knew my purpose.

So don't sit there and expect me to waste you Sammy, because I'm only ganking you if I can take me with you. Heaven knows what protection you might need in the next life – so don't think you can get away that easily.

And yeah, I suppose if there is a demon in your blood and we are both gonna die anyway, I guess now would be a good time to tell you that there isn't a gun to your head because I love you. And I would do anything for you.

More than anything I want you to have a normal life. Find romance, a family, a nine to five job if that's what keeps you safe. But it's too late for that Sammy – this is all I have left to give you: my promise that I am not walking away, and if I can't save you I pray to whoever is listening that I can go with you.

You hear me Sammy?

And Dean looked up at Sam and opened his mouth to say one or any of these things, when they were interrupted and the moment was lost – just like so many before.