In the majority of cases excessive breast development is caused by excessive fat deposit levels, but on occasion, it can be caused either by hormonal problems or as a side effect by those taking oestrogen to treat cancers or steroids in some cases.
This can develop among teenagers as most have some breast development at puberty, and is often uncomfortable and can cause them to become very self-conscious and lose some of their body confidence. For most young men, it regresses naturally over a few years as they grow out of puberty. However, there are many cases where it does not regress, and the enlarged breasts will remain. Other times, only the nipple may reman enlarged, this is commonly known as 'Puffy Nipple'.
This condition is also very common with older men. As their bodies begin to change, they produce less testosterone, while at the same time many older men tend to have more body fat which causes the body to begin producing more oestrogen. All these changes in the hormones can cause excess tissue growth in the breasts. Losing weight may help in some cases, however, the breast tissue may not improve through this alone.