According to self-determination theory (SDT), relationships that support the needs for autonomy, competence, and relatedness are essential to youth development. It is yet unknown whether adolescents differ in what constitutes “optimal” supplies of support. This research proposes methodological extensions to analyze how the degree of fit or misfit between needs versus supplies in support (i.e., from teachers, parents, and peers) relates to engagement in class, academic achievement, and well-being. Data were collected on 389 adolescents (M(age) = 14.3, SD(age) = 2.1, 58% female, low to high SES). Extending SDT’s contention that “the more supplies the better”, results suggest that the impact of supplies actually depends on the level of needs, and that commensurate (for parents) or surplus supplies (for teachers, peers) are most optimal. Therefore, while some youth require strongly supportive relationships to experience optimal development, others require much lower support, and may even suffer from higher support.