Some theories: - Food is a major component of identity. It conveys things about your values and belief systems. Thus, like other identity factors (race, religi

Some theories: - Food is a major component of identity. It conveys things about your values and belief systems. Thus, like other identity factors (race, religion, political position, sexual orientation, socio-economic class), when this is challenged, some people are particularly sensitive and respond very defensively. - As food choices are widely accepted to be a major factor in overall health (and thus, susceptibility to serious illness, life-expectancy, etc), it's natural for there to be a significant amount of fear behind people's positions in the discussion. I.e., when someone says "this diet will give you a greater chance of positive health outcomes than the one you currently follow", that can be frightening to people who harbour fears - conscious or subconscious - about their health and their expected lifespan.