Male causes of infertility are usually poor sperm quality. 
POOR SPERM QUALITY CAN BE CAUSED BY:

• Lacking sperm production in the testicles 
• Non-descended testicles 
• Infections 
• Fever
• Testicle cancer 
• Antibodies in the sperm cells

Apart from the reasons cited above, there are other factors like heavy smoking, alcoholism, irradiation from occupational hazards, overtight clothes, extreme heat and exertion etc. Again, like in women, some men suffer from unexplained infertility.