The main causes of mastitis are as follows: First, the accumulation of milk is due to improper breastfeeding techniques or incidental contact with the mammary glands by the child, which causes the milk ducts to close and ultimately leads to the accumulation of milk. Milk is an ideal culture medium that facilitates bacterial growth, leading to mastitis. Second, bacterial invasion occurs when there are cracks or abrasions on the nipple, allowing bacteria to travel along the milk ducts to reach and infect the mammary lobules. Lastly, a decrease in the patient's own immune system can also lead to bacterial infection.