I don’t think we view this quite the same way. I don’t see a problem with something being a defining part of someone’s identity, because it’s not up to me to decide how other people ought identify. We all have identities so on some level everyone identifies with something. Just because say, you and I don’t get how diet can be an significant part of someone’s identity, doesn’t invalidate it being significant for someone else. It just means that we haven’t lived the same life and that’s fine.

In a way we do have to accept other people’s identities, because what else can we do? We can’t force change on others, all we can really do is acknowledge the situation and then choose whether or not we want to continue interacting with them.

It doesn’t really get problematic until you have someone that does try to force things on others. My mother was one of those militant vegans, and she lost a lot of friends for it. She’s the reason I am vegetarian, and the reason I was vegan for a long time. Back then I never had much choice in either regard.

Being vegan wasn’t even really a defining part of her identity. Her trying to force it on others was also less about trying to change others, and more about trying to place herself in some sort of morally superior position to them. She pretended to care about their health and the environment - hell on some level maybe she genuinely did care - but it really was more about making herself look and feel better about herself because she was (and is) a deeply insecure person pretending to be otherwise.

As a final aside; what I’ve written above is what I consciously believe, but I’m obviously not an infallible person. I’m an extremely cynical, nihilistic, and definitely a judgmental person. I try my best not to be, and to see things from the perspective of others, but some days I succeed better than others.