But is it driven by a desire for the diet, or a desire against things that provoke an undesirable reaction?

I am forced into an extremely limited diet to avoid provoking my body any more than I have to. And, notably, one of the first reactions used to be something not tasting as good as it used to (or in one case tasting worse than it had.) It doesn't always happen but when it does it's a near 100% accurate test--the only time it ever fooled me the actual culprit turned out to be something my wife put on her face.

I saw what happened to my mother (very similar path, but started much later in life), I already knew how to isolate what was giving me trouble before it ever happened. Most people don't, though, especially when dealing with things where it isn't high on the ingredients list (or, sometimes, not at all--they are strong about requiring manufacturers to list what they put in, but there is no such requirement about noting what they fail to take out from a natural source. Not to mention being allowed to specify that most evil of ingredients "artificial flavors". The second most evil being "natural flavors.")