There are 6 reasons for the occurrence of breast cancer as follows: 1. Genetic factors: If immediate family members such as mothers, sisters or daughters have breast cancer, the chance of breast cancer for oneself will increase 3-5 times; 2. Reproductive factors: The chance of breast cancer will increase greatly if the age of first menstruation is less than 12 years old, the age of menopause is later than 55 years old, the age of first full-term pregnancy is older than 30 years old, or if one has not had children; 3. Hormonal factors: long-term use of birth control pills or estrogen-containing foods and drugs; 4. Nutritional factors: long-term excessive alcohol consumption or long-term consumption of high-fat diet; 5. Environmental factors: patients with a history of extensive ionizing radiation or radiotherapy, such as survivors of atomic bombings; 6. patients with a history of atypical breast hyperplasia, obesity, etc. In conclusion, the development of breast cancer is the result of multifactorial long-term effect. Knowing the above factors and consciously avoiding them can prevent the occurrence of breast cancer before it happens.