Male infertility has many causes, including medical causes, environmental causes, and lifestyle causes. Human reproduction is a complicated process, one in which any number of things can go wrong. In order for a male to successfully impregnate his partner, there are several crucial steps that must happen: healthy sperm must be produced, sperm must then be transported from the testicles and mixed with semen prior to ejaculation, there must be a high enough number of sperm in the semen, and the sperm must be functioning and motile.