It's logic, a simple kind of logic that Castiel's life has sorely been lacking, and he can cling to it for now. He can hold on to logic and let that be his rule for now, in a life that has been devoid or rules for quite a while now. It allows him to move properly, so let Sam lead him.

Castiel doesn't go quickly, can't at this point. And he doesn't go with his head held high. But he goes, and he manages not to look back even when fresh air hits his face and Dean opens the Impala's backseat door for him.

When he carefully seats himself, it's with a soft, choked off sound of pain, and with that comes the refusal to let go of Sam's sleeve.