What Causes Endometriosis?

I had a laparoscopy last month and was diagnosed with severe endometriosis. The doctor said he couldn't get most of it because of its location. Most of it was attatched to blood vessels and my ureter. Now he wants to do Lupron injections for 6 months and if that doesn't work, I'll have to have a hysterectomy. Should I get a second opinion? I'm concerned about the safety of the Lupron injections.