Sam doesn't eat unless he has to while Dean's a demon. When he gets Dean back, the first thing his big brother notices is how skinny and weak he is. Dean goes into ultra protective mode and is so gentle and loving to Sam. He makes him food and when they have sex, Dean is gentle and he makes Sam feel so safe and secure again. Dean goes on and on about how his Sammy needs to be good and healthy, and how well he's doing as he keeps gaining weight. Dean supports him through all of jt.