Sam always liked to bite. Even he was a baby; his favorite was Dean's finger. Exceptional Dean's fingers. It hurt even before he grew teeth. Sammy bites the hand that feeds him. As he was growing up, when he began picking fights with Dean, he bite. As soon Dean began gain dominance, Sam shank in his choppers. It took a whole lot of beating to stop that intendance.
Jessica liked being bitten. When Sam nibbled at her collarbone, her back arched and breathing stop.
But Dean hates it, more a biter then the biteee. He likes the upper hand, and when Sam digs into him, he loses that. Yet Sam can't help taken a yip here and there. Like now for example. Dean is yell into his lap
"OI watch the teeth!!"