Dean remembers when Sam was small enough he could pick him up.
It was very convenient to be able to just hoist the kid around and set him down wherever Dean wanted him to be. He himself had experienced this technique from Dad. Dean was sitting on Dad's favorite spot of the couch? He was scooped up and deposited on the other side. A monster was barreling towards them? Dean was lifted in the air and tossed out of the way.
When Sam was real young, the place Dean most wanted him to be was usually right by Dean's side. He'd snatch Sam off the floor where he was pawing at some of their meager toys and tuck him under his arm in a chair. He'd lift a sleeping Sam from his bed and take him back to Dean's own. And when Dad was gone, for hours, for days, and fear stabbed at Dean's stomach and Sam's eyes got wider and wider in his face, Dean would pick Sam up and put him right on his lap.
It was one of his favorite feelings in the world, though Dean would never ever admit it, having Sammy snuggled against his chest, a little ball of solid warmth with messy hair and little-kid smelling skin. He would sometimes squirm at first but after a grumble or a cuff on the head from Dean, Sam would always settle right down, and Dean knew he was just as happy being held. Nothing could touch him when Dean held him like that. He was safe.
As Sam got older it got harder for Dean to pick him up, both because Sam was getting bigger and because he was learning to fight back. Lifting Sam out of the way and sticking him on the kitchen counters when Dean was trying to restring the crossbow at the table meant taking several hard kicks to the shins. Lifting Sam away from his book at night and tossing him on his bed involved flailing blows to Dean's neck and face.
He could still pull Sam into his arms, but it started taking illness or unconsciousness to get him to stay there. Except on those rare, delicate times that Sam made the first move, ignoring Dean's squawks as sharp joints dug into him, tucking increasingly longer limbs up and putting his head on Dean's shoulder almost as if he knew that exact spot that would get the most of his hair in Dean's mouth.
Pretty soon Dean's reasons for picking Sam up went from bedtimes and teasing to blood loss and broken bones. He couldn't lift the kid anymore, not after his second growth spurt when he grew nearly three-and-a-half billion feet in one night, but he could still haul Sam's ass up off the ground when he went down and so Dean did just that, every single time.
Dean couldn't hold him on his lap anymore either, because the genetic freak would never fit, and he was usually too wrapped up in sullenness and sarcasm to even allow Dean to put a hand on his shoulder. There were still the occasional times like in the hospital after the hunt in Duluth when Dean was laying immobilized and plaster-coated, and Sam climbed right up onto the bed with him and tucked himself into Dean's side, uncaring that there wasn't enough room and they were both well past the age of cuddling.
Dean picks Sam up now, even though he's heavier (dead weight) than ever, and he carries him from the mud to the car and from the car to the abandoned shack. He won't let Bobby help, because this is Dean's job, this has always been Dean's job.
He remembers when Sammy was smaller, so much smaller and Dean could just pick him up all the time. Any time. Every time.
And Dean promised, he promised, he was never, ever gonna let Sammy down.