Male infertility can have many different causes:

Hormonal problems happen when the body produces too much or too little of certain hormones. This can affect sperm production and a man’s ability to have children. Testosterone replacement treatment can stop the body from producing sperm and should not be used if you are trying to have a child.
Genetic disorders or chromosome changes can affect sperm production and fertility. If your sperm count is very low, your doctor may recommend blood tests to look for certain genetic conditions before starting fertility treatment.
Problems with sperm production can happen due to a condition called varicocele, where the veins above the testicle (most often on the left side) become enlarged. This can affect sperm quality and make it harder to have children. Treating a varicocele may improve sperm quality and the chances of a natural pregnancy.
Difficulties with getting or maintaining an erection, problems with ejaculation, or other problems in the genital area can affect a man’s ability to have children.
An infection of the urogenital tract can affect the organs involved in urine and reproduction, such as the kidneys, bladder and genitals. Your doctor may prescribe antibiotics to treat the infection, although treating the infection does not necessarily increase the chance of a natural pregnancy.
Problems with the immune system can affect fertility in some men.
Certain cancers, especially testicular cancer, and some cancer treatments, such as chemotherapy or radiotherapy, may affect fertility. Your doctor can discuss ways to protect your fertility, such as storing sperm before treatment.
Damage to sperm DNA may reduce the chance of natural pregnancy and may be linked to repeated miscarriages. A test called sperm DNA fragmentation testing can check for this type of damage.
Lifestyle and age can also affect male fertility. Being overweight, not being physically active, smoking, drinking large amounts of alcohol, and increasing age can all affect sperm quality and reproductive health.

Which applies to you?