I had never noticed this little second until I was giffing the whole scene and I had to separate this. Just look at Dean! Sam’s groaning, in terrible agony, and Dean actually winces as if he can feel his brother’s pain, and then he looks at Sam with a look of of ‘Are you okay, Sammy?’ in his eyes. ASDCJLKASD! It’s killing me!
I love this scene. It’s so clear from their expressions that they are remembering all the times they spent in the back seat together. Dean making sure Sammy is comfortable. Sam laying his head on Dean’s lap or shoulder going to sleep. I think it is also a reminder that they always had each other. And despite everything, things were alright as long as they were together, looking after each other.