Dean is four years older then sam and was always told from a young age to watch out of Sammy. When a demon killed their mother,it was dean who carried baby sam out of the house. Dean was tacked as sam's protecter and as a result views himself as such. Thus whenever their father john Winchester went out hunting dean would watch over sam. He would cook and care for sam as well as protect him while their father was out. One night in the motel room when dean got back there was a shtriga standing over sam dean pulled a gun out but didn't shoot. Their father showed up then and got rid of the creature.
