There are mammary glands in both men and women body however men breasts are normally small and undeveloped. During the period of puberty, breast growth starts in boys, just like girls, and this is normal.
However, this growth may be more than expected in some boys. This is the most common cause of gynecomastia.
The second most common cause is the increase in adipose tissue in the breast area due to weight gain. Weight gain makes the breasts look larger.
The third most common cause is drugs usage. Drugs for hormone therapy or unconsciously used for bodybuilding may cause breast enlargement. Furthermore, some medications, excess weight and alcohol consumption may cause gynecomastia.