Throughout the series, Sam was defined by his rebellion against the hunting life and his desire for a "normal" existence—represented early on by Stanford and Jessica. In " Swap Meat ," he is literally forced into that life, living as Gary, a teenager with parents, school, and a typical suburban environment.
Dean's failure to see the "wrongness" in Sam highlights how much their relationship had become strained by the weight of their burdens, making Dean vulnerable to a version of Sam that finally "played along" with him. The Dark Side of Identity and Agency Supernatural 5x12
When Sam tells Gary to "rebel a little," it highlights his own realization that his earlier rebellion might have been more about opposing John Winchester’s control than a genuine desire for normalcy. Dean’s Wistfulness and "The Brother He Wants" Throughout the series, Sam was defined by his