This paper reports on a case study of a student's rationales for learning mathematics. We operationalize Stieg Mellin-Olsen's educational concept of rationales for learning and apply the concept on data consisting of three semi-structured interviews with a student in the Swedish prison education program. Our analysis shows that the student's rationales vary in character over time as a reaction to his educational contexts. We conclude that Mellin-Olsen's construct of rationales is useful for understanding students' changing motivation in relation to the teaching and to the practice of mathematics the teaching entails. Teachers may use the concepts from our analysis as cognitive tools, related to students' rationales for learning. By identifying students' different rationales, opportunities arise for an individualized instructional design.