"I’m proud of us."
Let Dean’s words sink in, if you please. He’s not proud of his actions on his own. HE IS PROUD OF THEM TOGETHER, AND WHAT THEY’VE ACCOMPLISHED TOGETHER.
That’s his farewell to his lifelong friend, his brother, his Achilles heel. Sammy, who always calls shotgun, because Dean always picks driver and the music. He doesn’t tell him he loves him; Sam already knows that. Instead, he says he’s proud of what they’ve done as a team, what they’ve been through for each other and the rest of humanity, what their goals are and were.
Because, if Dean never comes back, he needs to tell Sammy that he’s proud, and that his spirit will be content and rest in peace. That’s what Sam needs to hear.
BRB, crying my eyes out.
Also, I'm gonna listen to MCR's "Welcome to the Black Parade" on loop, because that's eerily appropriate right now.