And I am my brother's keeper
On a night full of anger, they both say things they regret. Dean is hurting—inside and out—and Sam keeps pushing long after he knows he should stop. He shoves hard, and it's not even Dean he's pissed at.
It's never Dean he's pissed at, but Dean still always bears the brunt of his fury anyway.
And on that night—still aching from the asylum and Sam's half-assed apology and Dad's complete lack of an apology—Dean drives away. First time he's ever left Sam. Ever.
Sam watches him go, feeling more lost than he's ever been, because Dean isn't the one who leaves.
And if this is what it feels like, no wonder Dean was so pissed. Sam thinks he might understand his brother a little bit more. But not enough to call him back.