This article reports a comparative study on the reasons for pre-service student teachers becoming teachers in Hong Kong and Macau. The study adopted a mixed methods research design, with 459 and 137 student teachers completing a questionnaire on teaching motivation and 35 and 15 student teachers joining a follow-up interview in Hong Kong and Macau, respectively. The study found that student teachers in both societies share commonality in their altruistic and intrinsic motivations, i.e. a sense of vocation. The difference in student teachers' motivation reflects the contrasting perceptions of teachers' work and the teaching profession in connection with the different educational systemic factors in Macau and Hong Kong. Government-initiated quantitative expansion and quality enhancement in education set the backdrop of an aspiring teaching profession with promising prospects as perceived by the student teachers in Macau. In contrast, the move towards "centralized-decentralization" with a focus on excellence, competition and accountability in education at a time of teacher surplus results in a demanding profession with gloomy prospects as perceived by the Hong Kong student teachers. These differences add to our understanding of how the development trajectories of different education systems and teaching professions influence young people's motivation to become teachers in different societies. The study also provides insights for initial teacher education and offers suggestions for the education communities of the two societies.