I go through "phases" where I'll only eat specific foods (I eat like a toddler, seriously) and there are phases where I eat absolutely no fast food, and phases where I eat nothing but fast food.
When I eat a lot of fast food, it doesn't bother me. But when I go months without eating any fast food, a single hamburger will destroy my stomach. I learned that years ago, ands I think it's due to tolerance. When I eat a lot of fast food, my body builds up a tolerance and can handle it. But when I go months without eating processed, greasy crap, that single hamburger literally makes me sick. And yet, that somehow never dissuades me.