For me personal I think it was parasites and overuse of antibiotics. I always ate pretty healthy, though I did in take too much sugar in desserts. But my parents were good and feeding my bother and I healthy meals. I never really consumed alchol. Never liked it. I worked out and kept stress down. Though there was one point in my life that I had some emotional stress that might have made this problem worse.
What I did to figure it out what cause this for me personally was think about when I started having problems. I could pin point a time, not everyone can, but I was a picture of perfect health until I went swimming in a lake while camping. There was some problem with the water and we were not allowed to go in it ( I think it might have been e coli but not sure) but they tested it in the morning and the camp ranger guy told us it was now safe to go swimming. Only my father and I went swimming. We both were very sick when we got home. I was given three different types of antibiotics. Nothing helped, because it probably was not a bacteria that I had but most likely a parasite but it the problem seemed to go away. Of course it dint, it just seemed like that. I had digestive issues ever since.
I also born very premature, was a sea section birth, and was unable to be given breast milk. This all made me at risk for thing like candida because I did not have the beneficial bacteria. Many of my organs were not even developed, and my immune system was developed mostly outside my mother body. New research is finding that the immune system is directly realty to the digestive tract. Also when a baby goes through the birthing canal it swipes up beneficial bacteria there. And the benefits of breast milk most people know about.
If I were you just think about when you started to have problems and see if there is any glaring issues that happened at that time.
Hope this helps,