Why does sterility occur without apparent cause?

Antonio Forgiarini, gynaecologist at Next Fertility Valencia, tells us in this video why sterility occur without apparent cause:

Well, we don't know really the cause of the unexplained infertility, because if not, it wasn't an unexplained infertility. So, in these cases I think it's a very good option to do a fertility treatment so we can run more test and, especially, we can see in the laboratory what should happen in the body. So, we can control it and we can give a name to this sterility, a reason for the sterility.