Mama Said
For many people his age, memories of early childhood are the ones most likely to have long faded from memory. They're the ones that blur most with the passage of time, dulling over the years as newer ones take their place, while worries and responsibilities replace the simplicity and happiness of younger days. The older the memories are, the more they're distorted through the lens of a child's perception, so that the adult they belong to is never quite sure how accurate the recollection is. But accuracy isn't important. What matters is the comfort such memories can bring in a time when life has worn childhood innocence away.
And when happiness in childhood was so short lived, they're needed all the more.
Dean still remembers her. The warmth of her smile and the safety of being held in her arms, knowing that nothing could ever go wrong when she was around. Whenever she put him to sleep he was never scared of the monsters under the bed, because he knew she'd always be there to chase the nightmares and the demons away. It's more than Sam has, he knows, who's too young to even remember her face from anything but photographs, but sometimes he's glad that Sam being too young to remember Mary means he's too young to remember the night she died as well.
Dean remembers it all too clearly. He remembers the fire savagely burning its way through the house; remembers the smoke and the heat and the fear of it; remembers the terror of hearing his mother's screams, but he didn't see what happened to her before John put Sammy in his arms and told him to run. So he did, carrying his baby brother out of the door without looking back.
It took weeks for the nightmares to stop after the Yellow-Eyed Demon had paid them that fateful visit. Dean would wake crying and screaming out for his mom to come and save him from the bad dreams again; but the dream was real and Mary was gone, burnt to a crisp on the ceiling, and all Dean had was John telling him to be brave because he had to be for Sam. And so Dean learned to stop crying, and to not think of that night except to remind him that they had to get revenge. There wasn't time to grieve for Mary when they had a mission to find Yellow-Eyes.
Even now, with Azazel dead, and with John having joined Mary in the grave, it still isn't quite enough to bring closure. The nightmares still haunt him from time to time, having had plenty more horrors come to accompany them in their torment over the years. The memories of that night have permanently branded themselves on his soul; the first scar that laid the foundations for countless more to come.
He still has the memories of the times that came before that: comforting and deeply treasured, but far too few in number and tainted by everything that came after. Sometimes he still prefers to bury them so that he can bury everything else, because his adult self feels all to keenly the loss of the happy, normal life he'd once had.
He remembers the last words Mary ever spoke to him, loving and sincere, as if she truly believed them; "Angels are watching over you."
He wishes now that they could comfort and reassure him like they once did, but there's a cold, lonely gravestone somewhere in Kansas to remind him just how hollow and meaningless those words really are.