Dean had Mary’s eyes.
John hated himself for noticing it. When John had lost himself in whiskey in the months following Mary’s death, Dean’s blond hair had grown until John found himself staring, horrified, at his wife’s hair on his eldest child’s head. He couldn’t remember what happened after that, but the next day, he had woken up to discover Dean had shaved his own head.
Little Sammy looked like John. To John’s despair, he found himself doting on Sammy, losing himself in the comfort of Sammy’s dissimilarities to Mary. Sammy had his dark hair, his eyes, even his stubbornness. No one could ever mistake Sammy for anything but John’s son, especially when Sammy spoke.
In contrast, no one believed Dean was John’s son. As Dean grew, he tried to mimic John’s mannerisms, but John always saw Mary. As he doted on Sammy, he made constant demands of Dean. Mary was never a soldier, so Dean would be one.
Except Dean would smile, and John would still see Mary.
Except Dean would sing Sammy to sleep, and John would hear Mary’s lullabies.
Dean even tasted like Mary, coffee and chocolate, and he made the same sounds, and his hair was just as soft.
Dean only had one major difference from Mary; Mary had never looked at John with fear in her eyes.
John hated Dean for that.