The causes and etiology of nipple pain in the left breast are more numerous and require precise diagnosis, as follows: 1, too tight underwear: excessively tight underwear can produce strangulation of the chest and back, which in turn triggers nipple pain in the left breast; 2, localized tissue damage: localized tissue damage in the nipple region, such as nipple chafing, breaks, and injuries, which can also trigger nipple pain; 3, adverse mental emotions: excessive tension, anxiety, and even Depression, these adverse mental emotions, can also trigger nipple pain; 4, breast disease: breast hyperplasia, breast cystic degeneration, and even breast occupying disease, mastitis, breast hyperplasia, these mammary gland diseases, can also trigger nipple pain.