Immortal being the key word. He hadn't needed to worry about Cas dying because Cas couldn't die. As soon as they returned the souls to Purgatory, Cas was supposed to be fine. Now that he's human, though, it's a whole different story and Dean is intimately aware of the effects of too little sleep and too little food on the human body.

"You get sick and die on me, I am gonna kill you, that's what it is."