Dean should know him better than that by now. Sam was going to go out fighting. Whatever was killing him he was going to fight it for as long as he could. He still had two more trials to go. He couldn't roll over and give in yet. Besides, he was only hacking up blood - no biggie...
"Dude, you can't just kiss a guy like that and then tell them to go to sleep. Seriously, I know you. I know you're worried and I know you want to look after me and keep me safe. But you can't Dean. Not this time. I gotta do these trials myself. And I need you to understand that." And be ready for the inevitable death of his baby bro.