Gynecomastia is a condition which causes the breast tissue of men and boys to swell up. It usually happens when 2 hormones in the male system are out of balance. Although men are born with nipples and a small amount of breast tissue, breasts do not develop in men as they do in women. Males make the male hormone testosterone, but also make a little of the female hormone estrogen.
These hormones are usually in balance, and if these hormones go out of balance, and estrogen becomes dominant, breast swelling happens.