Background: Graduate teaching assistants (GTAs) play critical roles in undergraduate education. Past studies have examined various programs developed for GTAs, but little work exists that examines GTAs' personal experiences and perceptions in engineering education. To develop teaching environments and training programs that foster positive motivation in GTAs for teaching and improve the quality of their teaching, we need to better understand the factors that motivate them. Purpose: Situated in self-determination theory (SDT), the purpose of our study was to examine graduate students' motivation to teach by answering the research question, "What factors contribute to GTAs' competence, autonomy, and relatedness with regard to teaching?" We scoped the study broadly to include perceptions of events, occurrences, people, institutional structures, and demographics as potential factors. Design/Method: We employed exploratory interview methods influenced by critical realism to understand GTAs' experiences. Our sample contained 12 participants from five different universities. We used a combination of a priori codes and open coding to determine the factors. Results: We identified five factors that affect the needs posited by SDT: training, previous experience, appointment structure, students, and teaching colleagues. Each factor uniquely contributed to a GTA's teaching motivation. Conclusions: GTAs are teachers, but they are also students. We recommend that faculty and administrators consider the five factors we identified and their unique individualized effects on GTAs as students and teachers when designing teaching experiences. This focus will lead to increased motivation in teaching and, with time, increases in the quality of teaching in engineering.