I was in middle school with a guy who ate a tremendous amount of ketchup, I feel like it must have started as a shock value thing, but who knows why other people do things. A bottle a day for him at lunch was not unheard of.
He was not a healthy kid, dude was pretty overweight and had heartburn problems, again this was in middle school. I remember him having to leave school for a bit because of serious stomach problems and then when he came back he was on a super restrictive diet. I think I only ever saw him eating green vegetables and basic salads after that.