Disclaimer: I don't own Supernatural, Sam or Dean and never will
Sam was Dean's.
That meant in everything. When Sam was in trouble, Dean protected him. When Sam was sick, Dean worried. When Sam was sad, Dean tried to cheer him up. When Sam was happy, well hell, Dean was happy.
Dean watched amusedly as six-year-old Sammy tried to cool off the fresh-out-of-the-fire marshmallow on the end of the stick like Dean had taught him (by waving it up and down in the air vigorously).
Sam was happy so Dean was happy.
Because Sam was Dean's. All his achievements and mistakes were Dean's as well.
So when the older Winchester watched the flaming white puff fly off the stick and become a flaming puff projectile, he and Sam shared one brief glance before they both took off like the wrath of hell was at their heels because well…
That's kinda what a pissed-off, beard-covered-in-sticky-goo Bobby Singer was like.
And sure it was Sam's marshmallow
But Sam was Dean's.