There is concern that a growing number of individuals, especially adolescents, are diagnosing themselves with mental disorders. However, there has been limited empirical research into this phenomenon: why it might happen, what the costs and benefits might be, and what the implications are for anyone who is experiencing distress. To address this, this study used reflexive thematic analysis to explore attitudes toward self-diagnosis of mental disorders as expressed on the discussion website Reddit. From 1195 user comments, five themes were generated: (1) There is tension over who is the expert in diagnosis; (2) Self-diagnosis is a route to self-understanding in an inaccessible system; (3) Teenagers on social media are the problem; (4) Self-diagnosis can become self-fulfilling, and (5) Now no one is believed. Together, these themes highlight that there is considerable anger, derision, and criticism targeted toward people who self-diagnose with mental disorders, and that this is particularly targeted toward adolescents who self-diagnose on or as a result of social media. The findings have important implications for understanding how to support and validate people, particularly adolescents, who (sometimes accurately) use diagnostic language to express how they are feeling.