Because there is love in Dean patching him up, but what goes unmentioned again and again is how Sam shows his love.
His love letter is written by carving his chest wide open, by letting Dean reach inside to crack open his ribs and press his hand against his beating heart.
Sam's letter was scribbled by a blue pen that is almost out of ink, with trembling hands and smudged handwriting.
Check me for wounds, look at me, look inside me; I'm asking you to see what marks I carry. I'm showing you all of my mistakes. This motel bathroom has barely working lights. The bathtub is stained. This is a confessional. I'm telling you all of my sins. Do you see me? Do you see?
Dean's response comes in stitching the wide gap in Sam's torax closed, but it's not so he can heal. Dean sews closed flesh and bone to keep it between the two of them.
No one else will see you like this, he writes, just me. And then he pushes Sam's hair back, gives him a little smile from above.
I know all that, Dean continues, I always have. I love you either way. You're not stained. You haven't been ruined for me, Sammy. I see you, yeah, I'd recognize the beat of your heart in the palm of my hand. I don't care about your sins.
Sam is sitting on the toilet lid so he has to crane his neck upwards to look at Dean once his brother stands. The bathroom lights may not work properly, but at this angle, the fluorescent bulbs make a blinding white halo around his head.
Sam flips the paper because there's no more room to write on the other side, hotel notepads aren't exactly large.
Is this holy?, Sam writes, To show someone your worst and be answered with love? This is what God does, right?
And Dean, moving to wash his bloodied hands in the sink and get more gauze, doesn't laugh. He knows Sam's question is genuine, he's being serious, so he doesn't. Instead, he takes the paper from his baby brother's hands when he's back to being hunched over him and finishes their letter with a No, that's just how I'm with you, kid. before going back to cleaning the nasty scratch on his stomach.