Despite the recognized importance of teaching motivation and goals for successful second/foreign language teaching, past research has revealed limited insights into teachers' goals and underlying reasons. Understanding teachers' motivational profiles offers valuable insights for educating enthusiastic teachers and supporting those facing psychological challenges, ultimately enhancing students' language learning. As one of the initial studies, this study investigated language teacher achievement goal complexes (i.e., what goals teachers pursue and why), drawing on achievement goal theory and self-determination theory. A qualitative case study explored seven Japanese secondary school English teachers through interviews. Thematic analysis revealed that their goal complexes were predominantly mastery-oriented, influenced by wanting-to rather than needing-to reasons, with other goal complexes identified in fewer cases. These reasons for goal pursuit were influenced by personal and social factors. Teaching experience also played a critical role in shaping goal complexes. This study advances theoretical understandings of goal complexes by illustrating how motivational reasons and contextual factors interact to influence language teachers' goals. Practically, it highlights the need to foster mastery-oriented goal complexes in teacher education and professional development. These findings inform teacher training, school policies, and institutional support systems about approaches to enhancing teaching quality, job satisfaction, and well-being in language education.