It only ever happens when Dean's on the edge of losing himself – in the pleasure and the desperation of seeking that moment of oblivion, that moment in which he seems to finally forget all he's lost. A hoarse, whispered babble of praise and affection he'd never let himself utter in the light of day escapes his lips… and Cas clings to every soft, tender word.
"Cas… so good, Cas, so… so fuckin' gorgeous, and perfect, and hot, and… need you, Cas, need you so damn much…"
Dean's grip is a little too tight, his hands a little too greedy, desperate and grasping as they move, rough and calloused, over Cas's skin, as if searching frantically for something Dean's not sure how to find; but Cas drinks in the words like they're the sweetest, clear water, and he's dying of thirst, committing them to memory and welcoming the pain as Dean drives into him, drowning out his own misery by pouring out words of affection and desire that Cas knows he'd deny later if asked.
That's why Cas won't ask – won't press his luck, won't risk pushing away the one good thing he's got – the one thing left with any meaning in the pointless existence he lives now.
Dean's praise and affection are precious, and Cas holds onto them for all he's worth – which granted, isn't much these days. They're pure gold, to be treasured in this world of nothing but death and suffering and loss – the only thing that's worth anything anymore.
The only thing Cas has left to live for.
He only hopes – in these dark, secret moments that are all skin and heat and wordless feelings that ache inside him but won't ever be voiced – that somehow, he can manage to give Dean something to live for, too.