Muscle dysmorphia is primarily characterized by an excessive preoccupation that one's body is not muscular or lean enough. Muscle dysmorphia has shown clinical similarities with eating disorders. The present study aims to explore the psychosocial factors underlying muscle dysmorphia symptoms by referring to Stice's dual pathway model (1994), a theoretical model of eating disorders. Three hundred and eighty-six men were recruited to complete an online survey including questionnaires assessing social pressures to reach a muscular body and internalization of the muscular body, drive for muscularity, muscular-enhancing behaviors, negative affect, narcissistic traits, and symptoms of muscle dysmorphia. Path analyses showed that the original model has a good fit, without, however, confirming a significant relationship between the drive for muscularity and negative affect. Thus, social pressure to reach a muscular body and its internalization were associated to a drive for muscularity and then, to muscle dysmorphia symptoms. The drive for muscularity was indirectly related to symptoms of muscle dysmorphia through muscle-enhancing behaviors as well as negative affect (although, only for individuals with higher levels of narcissistic vulnerability). Results supported the adaptation of the Stice's model to explain muscle dysmorphia symptoms and underlined the possible influence of narcissistic vulnerability traits in this condition.