Sperms being the male gamete are indispensable for fertilization. Hence, any abnormalities in their nature or structure can cause infertility. Sperm number or sperm count is an important factor that governs fertility. And its degree is also important; in certain cases it is possible to achieve pregnancy, but the process takes longer. Sperm motility also has to be adequate for it to reach the egg. Furthermore, structural deformities also lead to infertility.
Any or all of these problems can be caused by irregular hormones, injury to the testicles, infection or even congenital defects. Certain conditions may also be hereditary.
Semen has to be deposited in the vagina for successful conception. Certain males cannot achieve this due to improper erections or premature ejaculation. If there is a blockage in the epididymis or the vas deferens (the tubes that carries the sperms from the testis to the urethra), it results in the semen being devoid of sperms. A vasectomy does the exact same thing. Infection and disease may also have the same effects.
Many males inherently produce antibodies against their own sperms, damaging them drastically in the process. Injury and infection of the testicles may also result in the production of such antibodies.