Endometriosis feed off estrogen. The estrogen-dependent endo lesions, if left unchecked, will begin to create their own estrogen.
I do want to clarify that the pill or any other form of HRT does not prevent the Endometriosis from growing. The reason it is used is to help stop the pain by stopping the periods.
I was on a low dose BCP (Marvelon) for 8 yrs continuously and did help with my pain. However it still continued to grow even after deep excision surgery.