I am wondering what causes gynecomastia?

Many possible causes: This condition can arise on its own or can be secondary to certain drugs (ie marijuana and many prescription meds) and also can be secondary to hormonal problems that may signal a bigger issue, including endocrine tumors. If you aren't simply overweight consider seeing your md. You need a testicular exam, review of your medications, and several blood tests that are not part of routine bloodwork.

Many but...: Gynecomastia comes from the greek meaning "gyne" meaning woman and  "mastos"  meaning breasts.  it refers to abnormal development of large mammary glands in males. Although there may be many “causes”, most cases are idiopathic (no specific cause). The use of certain medications, drugs, organ dysfunction (kidney or liver), hormonal imbalances etc. May also be causative.