When the breast tissues in men get enlarged due to hormonal changes, the process is called gynecomastia. It does not involve physical pain, but can lead to depression and mental anguish. The major cause of gynecomastia is the creation of an imbalance between the testosterone and estrogen hormones. Now, as you might know, estrogen is not a male hormone, but a female one, and this hormone is what causes the enlargement of breasts in men. Other problems which lead to the problem of gynecomastia are hypogonadism, obesity, hyperthyroidism, kidney failure, liver disease, and hypoandrogenism.
Gynecomastia does not happen equally in both breasts at all times. Sometimes, it only affects one breast. Gynecomastia can also happen in boys in their years of puberty when they undergo hormonal changes.