What are the causes of hypogonadism?

Several: Impossible to really go into all of them. Essentially, 2 main ways: either through primary testicular failure where testes are unable to function, or, secondary gonadal failure which results from pituitary/hypothalamic inability to stimulate testes. Anatomical defects, trauma, genetics, etc. Can cause either.

Depends. : There is primary and secondary hypogonadism. In men, primary hypogonadism is caused by testicular cancer, trauma, or genetic disorders. Secondary hypogonadism is commonly caused by depression, anxiety, obesity, head trauma, aging, or other chronic diseases.