This research paper investigates the dynamics of student motivation within the framework of Self-Determination Theory (SDT; Ryan & Deci, 2017), a robust psychological theory encompassing six mini-theories. It scrutinizes the interplay between individual learners and their educational environment, and their collective role in shaping motivation within formal education settings. The study focuses on discerning the motivating factors behind the enrollment of Vietnamese EMI students in a prominent public university located in Southern Vietnam, and the elements that impact the evolution of their motivation over time. Data collected from 339 students through open-ended surveys provide insights into how student motivation fluctuates throughout the EMI program, identifying key reasons that instigate these changes. Findings suggest that students' motivation is primarily driven by intrinsic goals and the psychological need for competence and connection. Although these students exhibit a strong autonomous orientation, the study underscores the potential of academic stressors to diminish their motivation at times.