Motivation plays a crucial role in learning, particularly when mastering a musical instrument. However, it remains a relatively unexplored area in Hungarian research. In this study, the researchers aim to investigate the relationship between social relations and various aspects of motivation in learning musical instruments, including the motivation to start learning, continue playing, and practice musical instruments regularly. The sample included 264 Hungarian musical instrument students, categorized into three age groups: (1) early adolescents: 9–13-year-olds; (2) mid-adolescents: 14–16-year-olds; (3) late adolescents: 17–19-year-olds. In the course of the cross-sectional data collection, the students filled out the motivation for learning music (MLM) questionnaire, while they were also asked to answer questions about social relationships and demographic data. The MLM questionnaire is based on the five dimensions of self-determination theory: amotivation, external regulation, introjection, identification, and intrinsic motivation. The measurement tool demonstrated reliability across all five dimensions. According to the results, there are significant differences between the age groups in the motivation dimensions, while no gender differences were observed. Based on the regression analysis, social relationships have an impact on learning motivation; however, the role and strength of incentives change with age. For early adolescents, parents exert the greatest influence, whereas teachers and friends play a more significant role for mid-adolescents, and friends become the most decisive influence for late adolescents. The results can be useful for professionals and parents as they can contribute to maintaining motivation.