Final answer:
The 15-year-old with type 1 diabetes is likely rebelling because he is trying to control aspects of his life given the strict limitations imposed by his condition, a natural part of adolescence where seeking autonomy is crucial.
Step-by-step explanation:
The most likely reason for the behavior of a 15-year-old son with type 1 diabetes mellitus who is rebelling against family rules and his required diet is that he needs to control whatever parts of his life he can. Adolescents with chronic illnesses, such as type 1 diabetes, often feel that so much of their life is regulated by their condition. Their teenage years are a period where seeking autonomy and establishing an identity are paramount developmental tasks. This need for control can manifest as rebellion against family and dietary rules, especially given the strict regime required to manage their condition. While it's possible that an irregular diet could contribute to mood fluctuations, and therefore to the behavior of the adolescent (fluctuating moods and behavior), without evidence of substance abuse or misunderstanding on the parents' part, the desire for control in their lives is the most logical explanation.