Little is known about why people decide to mentor in the context of science, technology, engineering, and math (STEM) learning. The purpose of this qualitative study was to identify the motivations of undergraduate student mentors working in an afterschool STEM program for underrepresented elementary schoolboys. We used self-determination theory (SDT) to explain why the undergraduate students decided to become mentors and, for some of them, to persist as mentors. We interviewed a sample of 16 mentors about their backgrounds and experiences over three semesters. The participants experienced intrinsic and extrinsic motivations to become and persist as mentors. Each mentor articulated more than one reason, suggesting their motivations are multifaceted. Some motivations did not fit well with SDT, which points to the underlying complexity of why people mentor and how mentors' backgrounds shape their motivations.