The causes of moles may be related to heredity, developmental defects, ultraviolet radiation and trauma. 1. Hereditary factors: the formation of moles has a certain relationship with hereditary factors, if parents have moles, then the chances of their children having moles will also increase. 2. Developmental defects: Genetic mutations occur during the development of the organism, triggering abnormal proliferation of skin tissue cells, which may lead to nevi. 3. Ultraviolet radiation: long-term exposure of the skin to ultraviolet radiation may trigger an increase in melanin production, thus inducing the appearance of nevi. 4. Trauma: When local skin is traumatized, pigmentation may remain after repairing the skin lesions, which may lead to accumulation of melanin and induce nevus. There may be other reasons for moles, so patients are advised to go to the hospital and consult a professional doctor to understand the relevant issues.