According to Dr. David Redwine, pioneer in endometriosis, we are born with it. It forms during embryogenesis and has been found in miscarried or aborted fetuses upon autopsy.
Dioxin exposure plays a huge role. Please read Mary Lou Ballweg's findings from the Endometriosis Association of America.
The most recent research says it’s an inflammatory condition cause by a dysfunctional immune system. Personally I think a lot of different things can cause that immune dysfunction (other chronic illnesses, genetics, trauma/stress, accute illness) but I’m not a Dr or researcher 😅😂 Heal Endo by Katie Edmonds is a great book that looks at the most recent research if you’re interested.
They say it could be environmental and/or hereditary
Some of the doctor's i came across says hereditary