Informed by self-determination theory, this study investigates the motivations behind students' voluntary pursuit of a master's thesis and the crucial role of supervisory support in their academic journey. The study also explores the impact of writing a master's thesis on their growth and well-being. Data were collected through interviews with 20 students from diverse disciplines in government-funded universities in Hong Kong. The interviews centred on critical themes such as students' motivations, expectations, feedback practices, supervisor-student relationships, challenges, and engagement levels. Thematic analysis revealed that the students were primarily driven by well-internalised and autonomous extrinsic motivations to write a master's thesis, with the aim of laying the foundation for a future PhD and achieving personal and professional development goals. The interaction between these motivations and the supervisory support, particularly in meeting students' needs for autonomy, competence, and relatedness, significantly contributed to their academic, professional, and personal growth and well-being. The implications of these findings for supporting students' engagement and development are discussed. Copyright © 2024 Routledge.