How do boys get gynecomastia?

PRL, E2...puberty: Hi. If boys have new bilateral gynecomastia at 19, there's a problem with prolactin or estrogen. See your doctor.

Gynecomastia: Approximately, half of all men have breasts that are enlarged, a condition known as gynecomastia. In some cases, the cause is heredity or hormonal activity within the body while anabolic steroid use can create this unwanted side effect.

Hormone activity: In puberty there is a surge in baseline hormonal activity & males have a small amount of estrogen in their system normally. Breast tissue in some males is more sensitive to even small amounts and up to half will have transient breast tissue enlargement during puberty.It usually fades within 6-12 mo.Persistance or development of breast tissue at other times suggests an abnormality.

There's no telling: Why one teenaged boy gets gynecomastia and the next one doesn't remains a minor mystery of medicine. Don't try to figure out "why". It needs to be treated surgically without delay. Adolescence is hard enough without a boy having to deal with this stuff.