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 things like mood, energy, and a man’s chance of having children.
Genetic disorders or chromosome defects happen when there are changes in a person’s genes or chromosomes, which can affect their health or development. These changes can be inherited from parents or occur by chance.
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 your quality of sperm and make it harder to have children.
Difficulties with getting or maintaining an erection, issues with ejaculation, or other problems in the genital area can affect a man’s ability to have children.
An infection of the urogenital tract is when bacteria or germs cause problems in the organs involved in urine and reproduction, like the kidneys, bladder or genitals.
Problems with the immune system happen when the body’s defence system doesn't work properly. This can make it harder to fight off infections or cause the body to attack its own healthy cells.
Certain cancers - especially testicular cancer - and some cancer treatments (such as chemotherapy or radiotherapy) may affect fertility. Your doctor can discuss options for protecting your fertility, like banking your sperm, before treatment begins.