Medical universities have called for a professional dress code to preserve the dignity of the medical profession, creating a sense of respect, tranquility, and trust in healthcare recipients and improve patient safety. This study aims to explain the reasons behind poor adherence to the professional dress code by students of the dentistry school. A qualitative study was conducted to explain the viewpoints of dentistry students of Tehran University of Medical Sciences (TUMS). Twenty-three in-depth interviews with dentistry students of different genders, study years, living in different accommodations, and having different tuition payment status were conducted. Conventional content analysis was used to analyze the data. One hundred and twenty initial codes were extracted. They were categorized into common causes of non-adherence to the TUMS Dress Code and specific causes of not following a specific section of the dress code (hygiene, jewelry, and makeup sections). The codes of common causes were categorized into 4 main categories including defects in education, management shortcomings, changes in societal culture, and personal factors. All components of the educational system must be aligned with each other to overcome the barriers against the students’ adherence to professional dress and put forward appropriate interventions at the policymaking, regulatory and educational levels.