Because of the absolute or relative increase of estrogen level in the body, or because of the increased sensitivity of breast tissue to it. Both men and women secrete androgen and estrogen at the same time.

Only the difference in proportion reflects the gender difference, so any factor that leads to high estrogen level or decreased androgen secretion will cause this disease. Exercise more. Usually smear (Yunfuxiu) on the nipple and press it, which will shrink the nipple.