hi welcome to healthcaremagic   
  there are many causes which can cause it..like-                                                Drugs: Certain drugs, such as 
steroids and drugs to treat inflammation, 
high blood pressure, cancer, or 
depression may affect male fertility.
 Genetic: You may have been born with genes that may cause 
infertility. A gene is a piece of DNA that tells your body what to do or what to make. Genes may affect sperm production, movement, and transport.
 Hormones: Disorders in the testes may produce abnormal levels of hormones that may affect the production of sperm.
 Immunologic: An immune system problem may cause 
antibodies to be produced that may attack your sperm. The immune system is your body’s defense system against infections and diseases.
 Infections: Chronic (long-term) infections of the male reproductive system, particularly in the 
prostate, may affect male fertility.
 Lifestyle: Smoking, drinking alcohol, and using illegal drugs may cause problems with sperm production.
 Varicocele: A varicocele is a condition where the veins (blood vessels) in the 
scrotum are enlarged and dilated (widened). 
     so find out the cause and get treated