The two Winchesters’ final deaths aren’t what Cas expects.
When Sam dies, instead of feeling his core light fire, he just sees the brilliant light flicker and fade from his eyes. But, seconds later, he feels a big gaping hole get carved angrily from his chest, and his own soul. It crushes everything Castiel knows, forcing him to his knees to weep the loss of something /so beautifully and wonderfully made/.
When Dean dies, Castiel expects the flicker-fade thing that happened with Sam. Instead, when the older Winchester dies, the reaction is immediate. He feels the cracks and holes in his being fill again; he wants to weep with joy for how good it feels. Then, suddenly, its ripped away forcefully, taking even more of Castiel with it. This time, fallen angel has no tears left.
When he’s finally found after running an angry rampage on Earth for several years, he is relieved. He wants this constant pain to go away. He does not want to feel empty right down to the very core of his soul; so he thanks God silently when the angel blade pierces his skin.
It doesn’t hurt like the broken angel expects it to. It feels…satisfying. When he dies, its with a triumphant smile on his face. And when his soul leaves him, it sets a firework chain reaction through to the angels that surround his body, crushing them like the loss of the Winchesters did to him.