A client describes a lifelong pattern of behavior characterized by a significant lack of motivation for tasks they perceive as mundane or offering delayed gratification, such as administrative work or long-term academic projects. However, they report an ability to focus intensely on activities that provide immediate and powerful rewards, like competitive video games or high-stakes hobbies. Based on this specific pattern of motivation and reward-seeking, which neurochemical system is most likely functioning differently in this individual, and why?