Similar to damngoodcoffee’s perspective, my guess is mainly fear and insecurity. Ethical vegans’ motivations are pretty obvious but for the others, they don’t realize that they’re not their mom who died of cancer, their dad who had a heart attack, their grandparent with diabetes etc. and/or that they’re inherently lovable. Those who have watched loved ones suffer, or have experienced suffering themselves, may desperately defend their diet/protocol because it represents safety to them. Others who feel insecure about themselves may desperately defend their diet/protocol because they believe it will make them more attractive so that they can feel wanted and loved by someone, and even the ones who achieve their goal may still feel insecure about themselves and so they project their pain onto others like any other bully. I think this song holds a lot of truth: