When Castiel reaches into Sam and tries to touch his soul, it's both strange and familiar, pain nearly overwhelming him; Sam is glad when it's over.
It happens a second time, though at the time, Sam doesn't actually recall that it's happened before. That time, he's sort of barely conscious, not even able to move. He isn't sure what's going on, but he feels pressure, followed closely by a sense of pain that's greatly dwarfed by a soothing coolness spreading through him when he feels like he's burning up. He's not happy when it leaves, but he can't seem to muster up the energy to do anything before he's losing consciousness again. When Sam finally wakes, he's got more important things to worry about than why his whole torso feels like it's tingling in a really good way.
It doesn't come up again until the day that Cas finally brings his entire year-long stealth plan to fruition, taking the souls of Purgatory into his body and calling himself a god. Sam is mostly horrified by the situation, though he can't help but think that Castiel asking them to bow down and profess their love to him would be funny if things weren't so direly fucked up. Running to the rescue and trying to stop Castiel had temporarily distracted Sam from everything he'd been feeling, but when Cas turns to look at him, stares right into him, Sam abruptly feels like he's drowning in despair once more.
The things he's done in the past year without a care for anyone but himself weigh heavily on him, but they're nothing compared to the memory of endless torment at the hands of Michael and Lucifer, both of whom had been none too pleased with the guy who'd locked them both away. The only thing Sam had been glad for on that count was that they'd mostly left Adam alone.
Sam wants the memories to stop, wants to fall to the floor so he can curl in on himself and never move again, wants to die, but none of those are possibilities at the moment. Sam needs to be strong, keep it together until they can figure out what they're going to do about Castiel, if they can do anything at all.
I can take it all away, Sam.
Sam had been focused enough on trying to rise above the quagmire of his own emotions that he didn't notice whether Cas had spoken aloud or just in his head, but he feels engulfed by the words, anyway, a sudden sense of calm radiating through his entire being. Sam is caught up once again in staring at Castiel, though this time in amazement. He thinks he might have just started crying in sheer relief at the abatement of the feelings swamping him, but by then, physical sensations don't seem to matter very much.
Sam looks down for a moment, not sure what he can possibly say, the need to be strong for Dean warring with the need for what Castiel is offering him. He sees Cas' arm stuck halfway into his abdomen, and memories of the other times Cas has done this surface without warning. He remembers them both with crystal clarity, though he knows the second one, at least, should just be a vague recollection. Sam blinks slowly, unconsciously comparing the memories to the present and marveling that it doesn't hurt at all this time. He's not entirely sure what to make of that, or if it has any significance. All Sam knows now is that Castiel is touching his soul, and he's never felt as good as he does right now.
Castiel must be able to feel Sam's hesitation, because he speaks to Sam again, and this time, Sam looks up and pays enough attention to know that what he's hearing is not being said aloud.
Let me do this for you, Sam. You are mine already, and I believe you know it. All I want from you is your love, your devotion.
Sam can feel the truth in the words, his soul practically singing to him that it belongs to Castiel, that he belongs with Castiel. His gaze turns slowly away from Cas and toward Dean, who's obviously saying something, but Sam can't seem to hear anything other than the sound of his own soul reaching out to Castiel and Castiel's entire being reaching back. He finally hears his name fall from Dean's lips, sees Dean reach out to him, but he's certain that if he lets Dean touch him right now, all his memories and the accompanying pain will come flooding back to him. He's not sure he can survive that after what Castiel has given him a taste of.
Sam meets Castiel's eyes one last time, and he doesn't need to speak for Castiel to know that he's agreeing. Cas pushes his other arm inside Sam's belly, next to the first one, and Sam feels as if his soul is being opened wide, only instead of the shock of pain he'd been expecting, it's the most incredible thing he's ever felt. Better than any orgasm, better than loving Jessica, better than loving Dean. It feels like he's touching the face of God, and Sam can't help but wonder why they'd ever tried to put a stop this. Why would they want to crush something, someone as beautiful as Castiel?
Sam feels like he's soaring, and a glance at Dean reveals that his brother is crying, still reaching out to him, but unable to touch. Sam has but a second to wonder if his body will withstand this or if Castiel is going to take his soul up to Heaven or maybe into his own body, as he'd done with all the souls from Purgatory. He doesn't know, but he can feel that whatever's going to happen, it's starting now.
He's pretty sure he manages to mouth it before his whole world explodes into white light, and then all he knows is Castiel.