The first thing Sam is able to remember is light. Bright, bright light. Also he hears a snarling voice, cruel and horrendous followed by the scream of a woman.
The memory is quite blurry and for years Sam doesn't think about this small image, he has inside his mind.
It's the bright that fascinates him and also the reason why he can't place the scream. When he's younger he tries to remember the light, but the noise of the cruel voice makes and the screaming sound of the woman always disturbs the image.
Sometimes little Sam wishes the woman wouldn't scream so he can watch the light. He wants that light.
Later he realizes this must be the night, when Mary Winchester died. When Azazel came to bleed into Sam.
Sam feels guilty, because he's sorry that the woman who gave birth to him is dead but he still wants that light. This glorious fire and deep down in a very dark part of his soul he thinks Mary Winchester deserved to burn since she sold him long before he was even conceived.
It's a resentment against the world, who seems so determined to rip apart everything that bears the name Winchester. It's not defiance yet, because Sam wants to believe but the world obviously favours other people.
When Lucifer appears, he seems to burn every part of the world he touches.
He says Sam was made for him, made for to burn the world with him and Sam believes it.
Wants believe it, because Sam remembers the light he wanted in his childhood is the same as the light that Lucifer wraps around him now. In order to burn everything that is not them.
It feels good and familiar. Like something he lost a long time ago.
Sam wonders why.
The devil just smiles.
"You are what I desired to meet, despite the fact that I was locked deep and far away from everything. Just as I am what you have been living for and the world didn't want to have."
And then Lucifer lightens Sam up from the inside and the dark ignorant world with it.