Endometriosis begins to develop while you’re still in the womb.
Although a few recent studies have found displaced endometrioum-like cells outside of the uterine cavity of female fetuses, there is still a lot of research to be done. Several people now believe that those of us who have Endometriosis have had it since we were in our mothers’ wombs. Some believe it’s pre-ordained at birth where our Endometriosis implants will grow and at what Stage (1, 2, 3, or 4).
Unfortunately, there is still no proven cause of Endometriosis, so every “cause” is still just a theory. Without a known cause, a cure is difficult to even begin to develop.
Resources:
Reproductive Biomedicine Online
U.S. National Library of Medicine
Bloomin' Uterus 