He's seen it a hundreds times. A spouse or loved one loses something they go through the four stages.
Doesn't matter the supernatural element, people are real, normal and they always go through the same things.
First it's shock: "I can't believe their gone, they can't be. I love them so much, they can't be gone."
Then it's denial:"No, this isn't true. It can't be. I just talked to them. You're wrong!"
Anger comes soon after: "Why are you doing this to me! You bastards! Why didn't you save him!"
After that it's acceptance. Whether it's a matter of hours, days or weeks later, it comes: "The navie thoughts: He's in a better place, there was nothing we could have done. He knew we loved him."
Samual Winchesters knows these.
Not only because he witness them throughout his life but because he's experianced every single one of those emotions.
And He knows he's only a few months away from going through it all over again. His brother's about to die, after all.
The funny thing is he's not acting like he's seen other people or even how he reacted after Jess. But unlike with Jess, Sam doesn't feel like he's gone beyond shock.
Sure he's in denial about the brother that he's had all his life is about to leave him, but it's not really like he believes Dean would ever leave him.
And yes he's mad as fuck, he's furious with the demons with the world and mostly with Dean.
But acceptances?
Not happening.
Sam doesn't really know what he feels, only that acceptances isn't going to happen...ever. He's a Winchester after all and he's going to fight to the bitter end, the whole time clinging desperately to his brother, never once even considering letting him go.