Eyes
Dean sees everything. He watches his father's back when they hunt. He watches Sammy every other minute of the day. He can tell when the laundry needs doing, when the weapons need cleaning, when the money is running low. He can tell within a hair's breath the instant to fire. His aim with a knife is perfect. He can spot motion out of the corner of his eye moving faster than light or slower than a snail. He can see the fight coming before either of them does. The set of Sammy's shoulders, the pinch of John's forehead.
He wonders what the view is like from their seats. Must not be that great. Sammy doesn't see and Dad doesn't look.
Breath
Sam's voice carries the Latin rites through banishments, spells, exorcisms. His yells fill the house when there will be no soccer, no play, no school dance. His whisper keeps Dean from passing out, when the pain is huge and the blood is flowing. His explanations guide them through planning the next hunt, his navigating gets them from town to god forsaken town, his little boy smile gets them into where they need to go.
He reads out loud to himself when they are out hunting. He's alone, but he prefers it to the blood stench. The mud, the gunpowder, the salt fill his lungs and choke him.
Memory
It's so hard to look at Dean sometimes. He has her eyes, her smile and her way of picking them up and holding them together. He was altogether hers and he fell in love with that little boy in a way the could almost have rivaled his love of Mary. It breaks his heart to look at him. Especially now, especially now that he's a shadow of what he should have been.
He sees himself when he looks at Sam. Stubborn, noble, honest and selfish. It reminds him of being in the Marines, going after Mary, building his business, negotiating for their house, all with a single mindedness that was probably the death of a few people around him. He remembers raging at his youngest, his baby, he remembers the door slamming behind Sam, he remembers stolen glimpses from behind trees, of the man that was becoming.
He remembers Mary, everything about her. Her smell, her life, her laugh. And every time he remembers that night, he remembers why he does this .