It's not the first time they've been too late to save someone and it won't be the last. Dean hates to be the bearer of bad news, especially news like this.
"I'm sorry."
"Oh god. She was all alone," her mother sobs.
The words are out of his mouth before he can stop himself, "Everyone dies alone."
Sam gives him a look and Cas stares at him and the woman sobs and he just can't take it anymore, so he goes outside to wait in the Impala.
Later that night when Dean has given up falling asleep and resorted to staring at the ceiling and listening to Sam snore in the bed next to him, Cas appears by his feet.
"You're not alone," he says.
Dean doesn't reply, but Castiel stays with him until Dean falls asleep.