There are various causes of male infertility, and this list is not exhaustive.

Here are a few examples:

• low production (oligospermia) or total absence (azoospermia) of spermatozoa in the semen
• lack of testicular production
• obstruction of the ducts that allow sperm to be expelled
• sperm malformation (teratospermia)
• lack of sperm mobility (asthenospermia)
• premature ejaculation before vaginal penetration
• retrograde ejaculation, which sends sperm into the bladder
• cancer treatments such as chemotherapy and radiotherapy
• sexually transmitted diseases
• a man's age is also an important factor, although to a lesser extent than a woman's
• a genetic disease such as Klinefelter syndrome
• hypothalamo-pituitary/hypophyseal pathologies
• urogenital infections 
• exposure of the testicles to extreme heat