Providing professional development to in-service teachers remains a high priority when attempting to meet state and federal school accountability requirements, yet principals may neglect teacher motivation and the teacher change process when facilitating teacher learning. Drawing on self-determination theory, this research examined social and environmental factors shaping teacher motivation in the context of lesson study with open approach. This descriptive case study included the perceptions of eight secondary mathematics teachers who engaged lesson study as a form of professional development. Thematic analysis of semi-structured interviews suggested the following contextual factors improve motivation: emerging proficiency, synergy, interpersonal dynamics, volition, and internalization. Additionally, apprehension, minor barriers, and contentious interpersonal dynamics may reduce intrinsic-like motivation. As teachers encountered supportive contextual factors, they satisfied their need to experience competence, relatedness, and autonomy which encouraged value internalization of new learning manifesting as an epistemological shift in teaching paradigm. By internalizing the value of new learning, teachers may become more likely to authentically enact new pedagogy in their classrooms. The results of this study indicated lesson study with open approach may provide a robust form of learning capable of changing prior instructional beliefs. These findings suggest principals create and maintain contextual factors (with deliberate intent) that promote intrinsic-like motivation and the teacher change process to enhance the healthy development of teacher learning.